10-Q/A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q/A

Amendment No. 1

(Mark One)

x QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

FOR THE QUARTERLY PERIOD ENDED JUNE 30, 2013

or

¨ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

FOR THE TRANSITION PERIOD FROM                      TO                     

Commission file number: 001-35670

Regulus Therapeutics Inc.

(Exact name of registrant as specified in its charter)

Delaware	26-4738379
(State or Other Jurisdiction of Incorporation or Organization)	(I.R.S. Employer Identification No.)
3545 John Hopkins Ct., Suite 210, San Diego CA	92121
(Address of Principal Executive Offices)	(Zip Code)

858-202-6300

(Registrant’s Telephone Number, Including Area Code)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark whether registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer”, “accelerated filer”, “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Large accelerated filer	¨	Accelerated filer	¨
Non-accelerated filer	¨  (Do not check if a smaller reporting company)	Smaller reporting company	x

Indicate by check mark whether registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    ¨  Yes    x  No

As of August 12, 2013, the registrant had 41,330,745 shares of Common Stock ($0.001 par value) outstanding.

EXPLANATORY NOTE

This Amendment No. 1 on Form 10-Q/A (this “Amendment”) amends the Quarterly Report on Form 10-Q of Regulus Therapeutics Inc. for the quarter ended June 30, 2013, originally filed with the Securities and Exchange Commission on August 14, 2013 (the “Original Filing”). We are filing this Amendment solely to correct a clerical error on the facing page of the Original Filing which stated that we had 36,155,745 shares of Common Stock outstanding as of August 12, 2013. The correct number of shares of Common Stock outstanding as of August 12, 2013 is 41,330,745 shares, as reflected on the facing page of this Amendment.

In connection with the filing of this Amendment and pursuant to the rules of the Securities and Exchange Commission, we are including with this Amendment new certifications by our principal executive and principal financial officer.

Except as described above, no other changes have been made to the Original Filing. The Original Filing continues to speak as of the date of the Original Filing, and we have not updated the disclosures contained therein to reflect any events which occurred at a date subsequent to the filing of the Original Filing other than as expressly indicated in this Amendment.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

	Regulus Therapeutics Inc.
Date: August 15, 2013	By:	/s/ Kleanthis G. Xanthopoulos
		Kleanthis G. Xanthopoulos, Ph.D. President and Chief Executive Officer (Principal Executive Officer and Principal Financial Officer)

EXHIBIT INDEX

Exhibit Number	Description
31.1	Certification of the Principal Executive Officer and Principal Financial Officer pursuant to Rule 13a-14(a) or 15d-14(a) of the Securities Exchange Act of 1934.
32.1*	Certification of the Principal Executive Officer and Principal Financial Officer pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002.
101.INS**	XBRL Instance Document.
101.SCH**	XBRL Taxonomy Extension Schema Document.
101.CAL**	XBRL Taxonomy Extension Calculation Linkbase Document.
101.DEF**	XBRL Taxonomy Extension Definition Linkbase Document.
101.LAB**	XBRL Taxonomy Extension Label Linkbase Document.
101.PRE**	XBRL Taxonomy Extension Presentation Linkbase Document.
*	These certifications are being furnished solely to accompany this annual report pursuant to 18 U.S.C. Section 1350, and are not being filed for purposes of Section 18 of the Securities Exchange Act of 1934 and are not to be incorporated by reference into any filing of the Registrant, whether made before or after the date hereof, regardless of any general incorporation language in such filing.
**	Pursuant to applicable securities laws and regulations, we are deemed to have complied with the reporting obligation relating to the submission of interactive data files in such exhibits and are not subject to liability under any anti-fraud provisions of the federal securities laws as long as we have made a good faith attempt to comply with the submission requirements and promptly amend the interactive data files after becoming aware that the interactive data files fail to comply with the submission requirements. Users of this data are advised that, pursuant to Rule 460T, these interactive data files are deemed not filed and otherwise are not subject to liability.

EX-31.1

Exhibit 31.1

CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER AND PRINCIPAL FINANCIAL OFFICER

PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I, Kleanthis G. Xanthopoulos, Ph.D., certify that:

1. I have reviewed this Amendment No. 1 on Form 10-Q/A for the of Regulus Therapeutics Inc.;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this report;

4. I am responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

a. designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

b. evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

c. disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. I have disclosed, based on my most recent evaluation of internal control over financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

a. all significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financial information; and

b. any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: August 15, 2013

/s/ Kleanthis G. Xanthopoulos

Kleanthis G. Xanthopoulos, Ph.D.

President and Chief Executive Officer

(Principal Executive Officer and Principal Financial Officer)

EX-32.1

Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the quarterly report of Regulus Therapeutics Inc. (the “Company”) on Form 10-Q for the quarter ended June 30, 2013, as filed with the Securities and Exchange Commission (“SEC”) on August 14, 2013 and as amended by Amendment No. 1 on Form 10-Q/A filed with the SEC on August 15, 2013 (the “Report”), I, Kleanthis G. Xanthopoulos, Ph.D., President and Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that, to my knowledge:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d), as applicable, of the Securities Exchange Act of 1934, as amended; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Date: August 15, 2013

/s/ Kleanthis G. Xanthopoulos

Kleanthis G. Xanthopoulos, Ph.D.

President and Chief Executive Officer

(Principal Executive Officer and Principal Financial Officer)

The foregoing certification is being furnished solely pursuant to 18 U.S.C. Section 1350 and is not being filed as part of the Report or as a separate disclosure document.

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Investments (Tables)

6 Months Ended

Jun. 30, 2013

Summary of Short-Term Investments

The following tables summarize our short-term investments (dollars in thousands):

	Maturity (in  years)		Amortized cost		Unrealized					Estimated fair  value
					Gains		Losses
As of June 30, 2013
Corporate debt securities		2 or less	$	45,139	$	2	$	(72	)	$	45,069
Commercial paper		1 or less		1,498		—		—			1,498
Certificates of deposit		2 or less		9,930		—		—			9,930
Debt securities of U.S. government-sponsored agencies		1 or less		7,050		—		(2	)		7,048
Total			$	63,617	$	2	$	(74	)	$	63,545
	Maturity (in years)		Amortized cost		Unrealized					Estimated fair value
					Gains		Losses
As of December 31, 2012
Corporate debt securities		2 or less	$	44,898	$	7	$	(49	)	$	44,856
Commercial paper		2 or less		6,492		—		—			6,492
Certificates of deposit		1 or less		6,200		—		—			6,200
Total			$	57,590	$	7	$	(49	)	$	57,548

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Condensed Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenues:
Total revenues	$ 4,759	$ 3,309	$ 7,997	$ 6,653
Operating expenses:
Research and development	7,722	4,883	14,604	9,487
General and administrative	1,723	984	3,628	1,905
Total operating expenses	9,445	5,867	18,232	11,392
Loss from operations	(4,686)	(2,558)	(10,235)	(4,739)
Other income (expense):
Interest and other income	63	27	135	54
Interest expense	(18)	(90)	(18)	(184)
Loss from valuation of convertible note payable	(2,697)		(4,458)
Loss before income taxes	(7,338)	(2,621)	(14,576)	(4,869)
Income tax (benefit) expense	10	(22)	1	(22)
Net loss	(7,348)	(2,599)	(14,577)	(4,847)
Other comprehensive loss:
Unrealized gain (loss) on short-term investments, net	(44)	(32)	(29)	34
Comprehensive loss	(7,392)	(2,631)	(14,606)	(4,813)
Net loss per share, basic and diluted	$ (0.20)	$ (10.78)	$ (0.41)	$ (23.46)
Shares used to compute basic and diluted net loss per share	35,994,642	241,223	35,933,961	206,610
Strategic Alliances and Collaborations
Revenues:
Total revenues	$ 4,759	$ 3,309	$ 7,997	$ 6,653

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Convertible Notes Payable

6 Months Ended

Jun. 30, 2013

Convertible Notes Payable

5. Convertible Notes Payable

2012 Amendment of the 2010 GSK Note

In July 2012, we amended and restated our 2010 GSK Note which resulted in the Post-IPO GSK Note. The amended and restated 2010 GSK Note was then simultaneously cancelled and our obligations thereunder terminated.

In October 2012, in conjunction with our initial public offering, we issued the Post-IPO GSK Note in the principal amount of $5.4 million, which was equivalent to the original principal amount of $5.0 million plus accrued but unpaid interest of approximately $0.4 million. The Post-IPO GSK Note has a maturity date of October 9, 2015. At GSK’s option, the Post-IPO GSK Note may be converted into shares of our common stock at any time prior to the maturity at a conversion price per share equal to the initial public offering price of $4.00, subject to complying with certain threshold ownership percentage limitations set forth in the Post-IPO GSK Note. At June 30, 2013, the fair value of the Post-IPO GSK Note is approximately $14.6 million, and is classified as “Convertible note payable, at fair value” on the condensed balance sheet.

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Net Loss Per Share - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	Jun. 30, 2013
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Convertible notes payable	$ 5.4
Conversion price per share of convertible notes payable	$ 4.00

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Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2013

Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis

The following table presents our fair value hierarchy for assets and liabilities measured at fair value on a recurring basis at June 30, 2013 and December 31, 2012 (in thousands):

	Fair value as of June 30, 2013
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents	$	18,401	$	18,401	$	—	$	—
Corporate debt securities		45,069		—		45,069		—
Commercial paper		1,498		—		1,498		—
Certificates of deposit		9,930		—		9,930		—
Debt securities of U.S. government-sponsored agencies		7,048		—		7,048		—
	$	81,946	$	18,401	$	63,545	$	—
Liabilities:
Convertible notes payable	$	14,592	$	—	$	—	$	14,592

	Fair value as of December 31, 2012
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents	$	39,363	$	39,363	$	—	$	—
Corporate debt securities		44,856		—		44,856		—
Commercial paper		6,492		—		6,492		—
Certificates of deposit		6,200		—		6,200		—
	$	96,911	$	39,363	$	57,548	$	—
Liabilities:
Convertible notes payable	$	10,134	$	—	$	—	$	10,134

Changes in Estimated Fair Value of Convertible Notes Payable

Changes in the estimated fair value of convertible notes payable from December 31, 2012 through June 30, 2013 are as follows (in thousands):

	Fair Value Measurements Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2012	$	10,134
Change in estimated fair value of convertible notes payable		4,458
Balance at June 30, 2013	$	14,592

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Pro forma Net Proceeds From Public Offering (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Subsequent Event [Line Items]
Gross proceeds (including over-allotment)	$ 49,163
Underwriting discounts and commissions	(2,950)
Estimated total offering costs	(360)
Offering costs paid as of June 30, 2013	0	(110)
Pro forma net proceeds	$ 45,853

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Changes in Estimated Fair Value of Convertible Notes Payable (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in estimated fair value of convertible notes payable	$ (2,697)	$ (4,458)
Fair Value, Inputs, Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		10,134
Change in estimated fair value of convertible notes payable		4,458
Ending Balance	$ 14,592	$ 14,592

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Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	$ 81,946	$ 96,911
Convertible Notes Payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis	14,592	10,134
Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	18,401	39,363
Corporate Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	45,069	44,856
Commercial Paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,498	6,492
Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	9,930	6,200
US Government-Sponsored Agencies Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	7,048
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	18,401	39,363
Fair Value, Inputs, Level 1 | Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	18,401	39,363
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	63,545	57,548
Fair Value, Inputs, Level 2 | Corporate Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	45,069	44,856
Fair Value, Inputs, Level 2 | Commercial Paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,498	6,492
Fair Value, Inputs, Level 2 | Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	9,930	6,200
Fair Value, Inputs, Level 2 | US Government-Sponsored Agencies Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	7,048
Fair Value, Inputs, Level 3 | Convertible Notes Payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis	$ 14,592	$ 10,134

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Revenue from Strategic Alliance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenue from External Customer [Line Items]
Total revenues	$ 4,759	$ 3,309	$ 7,997	$ 6,653
Strategic Alliances and Collaborations
Revenue from External Customer [Line Items]
Total revenues	4,759	3,309	7,997	6,653
Strategic Alliances and Collaborations | GSK
Revenue from External Customer [Line Items]
Total revenues	1,303	809	1,489	1,623
Strategic Alliances and Collaborations | Sanofi
Revenue from External Customer [Line Items]
Total revenues	2,905	2,500	5,406	5,030
Strategic Alliances and Collaborations | Astra Zeneca
Revenue from External Customer [Line Items]
Total revenues	465		929
Strategic Alliances and Collaborations | Biogen Idec
Revenue from External Customer [Line Items]
Total revenues	$ 86		$ 173

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Stock Option Activity (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Number of options
Options outstanding at December 31, 2012	4,720
Granted	340
Exercised	(223)
Canceled/forfeited/expired	(190)
Options outstanding at June 30, 2013	4,647
Weighted average exercise price
Options outstanding at December 31, 2012	$ 2.11
Granted	$ 7.81
Exercised	$ 0.91
Canceled/forfeited/expired	$ 3.33
Options outstanding at June 30, 2013	$ 2.53

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Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Amortized cost	$ 63,617	$ 57,590
Unrealized Gains	2	7
Unrealized Losses	(74)	(49)
Estimated fair value	63,545	57,548
Corporate Debt Securities
Investment [Line Items]
Maturity	2 or less	2 or less
Amortized cost	45,139	44,898
Unrealized Gains	2	7
Unrealized Losses	(72)	(49)
Estimated fair value	45,069	44,856
US Government-Sponsored Agencies Debt Securities
Investment [Line Items]
Maturity	1 or less
Amortized cost	7,050
Unrealized Losses	(2)
Estimated fair value	7,048
Commercial Paper
Investment [Line Items]
Maturity	1 or less	2 or less
Amortized cost	1,498	6,492
Estimated fair value	1,498	6,492
Certificates of Deposit
Investment [Line Items]
Maturity	2 or less	1 or less
Amortized cost	9,930	6,200
Estimated fair value	$ 9,930	$ 6,200

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Basis of Presentation and Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2013

Basis of Presentation and Summary of Significant Accounting Policies

1. Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In management’s opinion, the accompanying financial statements reflect all adjustments, consisting of normal recurring adjustments, considered necessary for a fair presentation of the results for the interim periods presented.

Interim financial results are not necessarily indicative of results anticipated for the full year. These unaudited condensed financial statements should be read in conjunction with our audited financial statements and footnotes included in our Annual Report on Form 10-K for the year ended December 31, 2012, from which the balance sheet information herein for the year ended December 31, 2012 was derived.

On September 7, 2012, our board of directors approved a one-for-two reverse stock split of our common stock. The accompanying condensed financial statements and notes to the condensed financial statements give retroactive effect to the reverse split for all periods presented. No further splits (or reverse splits) of our common stock have been contemplated as of June 30, 2013.

Use of Estimates

Our financial statements are prepared in accordance with GAAP. The preparation of our financial statements requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions.

Revenue Recognition

As of June 30, 2013, we have ongoing strategic alliance agreements and collaborations with GlaxoSmithKline plc (“GSK”), Sanofi, AstraZeneca AB (“AstraZeneca”) and Biogen Idec MA Inc (“Biogen Idec”). Our revenues generally consist of upfront payments for licenses or options to obtain licenses in the future, research and development funding and milestone payments, as well as funding received under government grants. We recognize revenues when all four of the following criteria are met: (1) persuasive evidence that an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured.

For these multiple element arrangements, deliverables under our agreements are accounted for as separate units of accounting provided that (i) a delivered item has value to the customer on a stand-alone basis; and (ii) if the agreement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the control of the vendor. The allocation of consideration amongst the units of accounting under our strategic alliance agreements and collaborations are derived using a “best estimate of selling price” if vendor specific objective evidence and third-party evidence of fair value is not available.

Milestones

We recognize revenue from milestone payments when earned, provided that (i) the milestone event is substantive in that it can only be achieved based in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance and its achievability was not reasonably assured at the inception of the agreement, (ii) we do not have ongoing performance obligations related to the achievement of the milestone and (iii) it would result in the receipt of additional payments. A milestone payment is considered substantive if all of the following conditions are met: (i) the milestone payment is non-refundable; (ii) achievement of the milestone was not reasonably assured at the inception of the arrangement; (iii) substantive effort is involved to achieve the milestone; and (iv) the amount of the milestone payments appears reasonable in relation to the effort expended, the other milestones in the arrangement and the related risk associated with the achievement of the milestone. Any amounts received under the agreements in advance of performance, if deemed substantive, are recorded as deferred revenue and recognized as revenue as we complete our performance obligations.

Generally, the milestone events contained in our strategic alliance agreements and collaborations coincide with the progression of our product candidates from target selection, to clinical candidate selection, to clinical trial, to regulatory approval and then to commercialization. The process of successfully discovering a new development candidate, having it approved and ultimately sold for a profit is highly uncertain. As such, the milestone payments we may earn from our partners involve a significant degree of risk to achieve. Therefore, as a product candidate progresses through the stages of its life-cycle, the value of the product candidate generally increases.

Deferred Revenue

Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying condensed balance sheets. Amounts not expected to be recognized within the next 12 months are classified as non-current deferred revenue.

Stock-Based Compensation

We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period.

We account for stock options granted to non-employees other than members of our board of directors, which primarily consist of members of our scientific advisory board, using the fair value approach. Stock options granted to non-employees are subject to periodic revaluation over their vesting terms.

Fair Value Option

Applicable accounting policies permit entities to choose, at specified election dates, to measure specified items at fair value if the decision about the election is: 1) applied instrument by instrument, 2) irrevocable, and 3) applied to an entire instrument.

In July 2012, we amended and restated our $5.0 million convertible promissory note originally issued in February 2010 to GSK (“2010 GSK Note”), which was accounted for as a debt extinguishment of the original note. We elected to measure the 2010 GSK Note under the fair value option. Upon initial measurement, the difference between the carrying value of the original note and the fair value of the 2010 GSK Note was recorded as a loss on extinguishment of debt to non-operating earnings. Thereafter, any change to the fair value of the 2010 GSK Note is recorded as a gain (loss) from valuation of convertible note payable to non-operating earnings.

Recent Accounting Pronouncements

In February 2013, the FASB issued Accounting Standards Update No. 2013-02, Comprehensive Income (Topic 220): Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income (“ASU 2013-02”). This update will require companies to present information about amounts reclassified out of accumulated other comprehensive income and their corresponding effect on net income in one place and reference the amounts to the related footnote disclosures. Current accounting standards present this information in different places throughout the financial statements. ASU 2013-02 was effective for us for the six months ended June 30, 2013. The adoption of ASU 2013-02 had no impact on our financial condition, results of operations, or cash flows.

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Investments

6 Months Ended

Jun. 30, 2013

Investments

3. Investments

We invest our excess cash in commercial paper and debt instruments of financial institutions, corporations, U.S. government-sponsored agencies, and the U.S. Treasury. As of June 30, 2013, our short-term investments had a weighted average maturity of less than two years.

The following tables summarize our short-term investments (dollars in thousands):

	Maturity (in  years)		Amortized cost		Unrealized					Estimated fair  value
					Gains		Losses
As of June 30, 2013
Corporate debt securities		2 or less	$	45,139	$	2	$	(72	)	$	45,069
Commercial paper		1 or less		1,498		—		—			1,498
Certificates of deposit		2 or less		9,930		—		—			9,930
Debt securities of U.S. government-sponsored agencies		1 or less		7,050		—		(2	)		7,048
Total			$	63,617	$	2	$	(74	)	$	63,545
	Maturity (in years)		Amortized cost		Unrealized					Estimated fair value
					Gains		Losses
As of December 31, 2012
Corporate debt securities		2 or less	$	44,898	$	7	$	(49	)	$	44,856
Commercial paper		2 or less		6,492		—		—			6,492
Certificates of deposit		1 or less		6,200		—		—			6,200
Total			$	57,590	$	7	$	(49	)	$	57,548

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Stockholders' Equity

6 Months Ended

Jun. 30, 2013

Stockholders' Equity

6. Stockholders’ Equity

Shares Reserved for Future Issuance

The following shares of common stock are reserved for future issuance at June 30, 2013:

	June 30, 2013		December 31, 2012
Common stock options outstanding		4,646,541		4,661,549
Common stock options available for future grant		2,171,920		946,403
Employee Stock Purchase Plan		481,274		150,000
Convertible note payable		1,389,417		1,366,787
Total common shares reserved for future issuance		8,689,152		7,124,739

The following table summarizes our stock option activity under all stock option plans for the six months ended June 30, 2013 (in thousands):

	Number of options			Weighted average exercise price
Options outstanding at December 31, 2012		4,720		$	2.11
Granted		340		$	7.81
Exercised		(223	)	$	0.91
Canceled/forfeited/expired		(190	)	$	3.33
Options outstanding at June 30, 2013		4,647		$	2.53

Stock-Based Compensation

The following table summarizes the weighted average assumptions we used in our Black-Scholes calculations:

	Three months ended June 30,						Six months ended June 30,
	2013			2012			2013			2012
Employee Stock Options:
Risk-free interest rate		1.5	%		1.0	%		1.4	%		1.1	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected volatility		67.6	%		70.8	%		67.5	%		71.0	%
Expected term (years)		6.1			6.1			6.1			6.1

The following table summarizes the allocation of our stock compensation expense (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
Research and development	$	571	$	121	$	1,070	$	183
General and administrative		188		84		476		135
Total	$	759	$	205	$	1,546	$	318

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Fair Value Measurements

6 Months Ended

Jun. 30, 2013

Fair Value Measurements

4. Fair Value Measurements

We have certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements.

Applicable accounting guidance defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants as of the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The guidance provides an established hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs that market participants would use in valuing the asset or liability and are developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our assumptions about the factors that market participants would use in valuing the asset or liability. The guidance prioritizes the inputs used in measuring the fair value into the following hierarchy:

• Level 1 includes financial instruments for which quoted market prices for identical instruments are available in active markets.

• Level 2 includes financial instruments for which there are inputs other than quoted prices included within Level 1 that are observable for the instrument such as quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets with insufficient volume or infrequent transactions (less active markets) or model-driven valuations in which significant inputs are observable or can be derived principally from, or corroborated by, observable market data.

• Level 3 includes financial instruments for which fair value is derived from valuation techniques in which one or more significant inputs are unobservable, including management’s own assumptions.

The following table presents our fair value hierarchy for assets and liabilities measured at fair value on a recurring basis at June 30, 2013 and December 31, 2012 (in thousands):

	Fair value as of June 30, 2013
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents	$	18,401	$	18,401	$	—	$	—
Corporate debt securities		45,069		—		45,069		—
Commercial paper		1,498		—		1,498		—
Certificates of deposit		9,930		—		9,930		—
Debt securities of U.S. government-sponsored agencies		7,048		—		7,048		—
	$	81,946	$	18,401	$	63,545	$	—
Liabilities:
Convertible notes payable	$	14,592	$	—	$	—	$	14,592

	Fair value as of December 31, 2012
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents	$	39,363	$	39,363	$	—	$	—
Corporate debt securities		44,856		—		44,856		—
Commercial paper		6,492		—		6,492		—
Certificates of deposit		6,200		—		6,200		—
	$	96,911	$	39,363	$	57,548	$	—
Liabilities:
Convertible notes payable	$	10,134	$	—	$	—	$	10,134

Changes in the estimated fair value of convertible notes payable from December 31, 2012 through June 30, 2013 are as follows (in thousands):

	Fair Value Measurements Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2012	$	10,134
Change in estimated fair value of convertible notes payable		4,458
Balance at June 30, 2013	$	14,592

We obtain pricing information from quoted market prices or quotes from brokers/dealers. We generally determine the fair value of our investment securities using standard observable inputs, including reported trades, broker/dealer quotes, bids and/or offers.

In July 2012, we amended and restated the 2010 GSK Note, which resulted in a debt extinguishment for accounting purposes. Concurrently with the debt extinguishment, we elected the fair value option for the 2010 GSK Note. The amended and restated 2010 GSK Note provided for a rollover of the 2010 GSK Note into a new promissory note effective as of the closing date of a qualifying initial public offering (“Post-IPO GSK Note”). We used a third party valuation firm to value the Post-IPO GSK Note at June 30, 2013 and recorded a loss from the change in valuation of convertible notes payable of $2.7 million and $4.5 million on the condensed statements of operations and comprehensive loss for the three and six months ended June 30, 2013.

The third-party valuation firm used an income approach in the form of a convertible bond valuation model to value the note. The convertible bond model considered the debt and option characteristics of the note. The key inputs to the model as of June 30, 2013 were volatility (70%), risk-free rate (0.49%), and credit spread (8.8%). The volatility inputs were based on historical and implied volatility of peer companies. Peer companies were materially consistent with those used previously in our 409A analyses and in previous valuations of this instrument. The risk-free rate inputs were based on the yield of US Treasury Strips as of each date. The credit spread inputs were based on a creditworthiness analysis of the Company and market rates for comparable straight debt instruments.

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Fair Value Measurements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Loss from valuation of convertible notes payable	$ (2,697)	$ (4,458)
Fair value assumption, volatility rate		70.00%
Fair value assumption, risk free rate		0.49%
Fair value assumption, credit spread		8.80%

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Assumptions Used to Estimate Fair Value of Stock Options Using Black-Scholes Option Pricing Model (Detail) (Employee Stock Option)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Employee Stock Option
Employee Stock Options:
Risk-free interest rate	1.50%	1.00%	1.40%	1.10%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected volatility	67.60%	70.80%	67.50%	71.00%
Expected term (years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

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Subsequent Events - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended		1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jul. 22, 2013 Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock			5,175,000
Issuance of common stock, per share			$ 9.50
Proceeds from public offering of common stock	$ 289	$ 34	$ 49,200

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Condensed Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	36,081,615	35,831,808
Common stock, shares outstanding	36,081,615	35,831,808

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Subsequent Events

6 Months Ended

Jun. 30, 2013

Subsequent Events

9. Subsequent Events

Public Offering

The unaudited pro forma balance sheet information below assumes the following transactions were completed subsequent to June 30, 2013 had occurred as of June 30, 2013:

• On July 22, 2013, we completed a public offering whereby we sold 5,175,000 shares of common stock at $9.50 per share and received gross proceeds of $49.2 million (before underwriting discounts and commissions and estimated offering costs not yet paid as of June 30, 2013).

Pro forma net proceeds from our public offering were determined as follows (in thousands):

Gross proceeds (including over-allotment)	$	49,163
Underwriting discounts and commissions		(2,950	)
Estimated total offering costs		(360	)
Offering costs paid as of June 30, 2013		—
Pro forma net proceeds	$	45,853

The following table summarizes certain actual balance sheet data and pro forma balance sheet data to reflect the activities related to our public offering noted above, as of June 30, 2013 (in thousands):

	June 30, 2013		Pro forma June 30, 2013
Cash, cash equivalents and short-term investments	$	82,715	$	128,709
Other assets		404		185
Accounts payable and accrued expenses		3,109		2,890
Common stock		36		41
Additional paid-in capital		124,363		170,352
Total stockholders’ equity	$	49,321	$	95,315

Amended and Restated License and Collaboration Agreement

In August, 2013, we entered into an amendment to the Amended and Restated License and Collaboration Agreement with Isis Pharmaceuticals, Inc. and Alnylam Pharmaceuticals, Inc. (“Alnylam”) dated January 1, 2009, as amended in June 2010 and October 2011 (as amended, the “Agreement”). Under the terms of the Amendment, the parties agreed to our use of certain Alnylam-controlled intellectual property concerning the use and manufacture of GalNAc conjugates (“GalNAc Process Technology”) on a non-exclusive basis. We will generally not be permitted to sublicense or otherwise transfer the GalNAc Process Technology and other Alnylam licensed intellectual property rights relating to GalNAc conjugate technology without the prior written consent of Alnylam, subject to certain limited exceptions for sublicenses to third party collaboration partners. There were no financial terms related to this Amendment.

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Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Operating activities
Net loss	$ (14,577)	$ (4,847)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	629	481
Loss from valuation of convertible note payable	4,458
Stock-based compensation	1,546	318
Amortization of premium on investments, net	638	233
Gain on investments	9
Loss on disposal of long-term assets	1
Deferred income taxes	(10)
Change in operating assets and liabilities:
Contracts and other receivables	(39)
Prepaid and other assets	(850)	(64)
Accounts payable	717	260
Accrued liabilities	160	(37)
Accrued compensation	(327)	117
Deferred revenue	(7,997)	(6,467)
Deferred rent and other liabilities	1,055	209
Net cash used in operating activities	(14,587)	(9,797)
Investing activities
Purchases of short-term investments	(19,984)	(7,075)
Maturities and sales of short-term investments	13,320	13,781
Purchases of property and equipment	(268)	(734)
Acquisition of intangibles	(96)	(130)
Net cash (used in) provided by investing activities	(7,028)	5,842
Financing activities
Proceeds from issuance of common stock, net	289	34
Principal payments on other long-term obligations	(56)	(216)
Costs paid in connection with initial public offering	0	(110)
Net cash provided by (used in) financing activities	233	(292)
Net decrease in cash and cash equivalents	(21,382)	(4,247)
Cash and cash equivalents at beginning of period	40,552	9,175
Cash and cash equivalents at end of period	19,170	4,928
Supplemental disclosure of cash flow information
Interest paid		22
Income taxes paid	1	206
Supplemental disclosure of non-cash investing and financing activities
Amounts accrued for property and equipment, net	1,025	29
Amounts accrued for patent expenditures, net	$ 19	$ 30

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Condensed Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 19,170	$ 40,552
Short-term investments	63,545	57,548
Contracts and other receivables	2,538
Prepaid and other current assets	1,620	829
Total current assets	86,873	98,929
Property and equipment, net	4,011	3,310
Intangibles, net	1,229	1,154
Other assets	404	125
Total assets	92,517	103,518
Current liabilities:
Accounts payable	1,028	311
Accrued liabilities	2,081	658
Accrued compensation	1,020	1,348
Current portion of deferred revenue	14,876	10,451
Total current liabilities	19,005	12,768
Convertible notes payable, at fair value	14,592	10,134
Deferred revenue, less current portion	7,834	17,756
Other long-term liabilities	1,765	767
Total liabilities	43,196	41,425
Stockholders' equity:
Common stock, $0.001 par value; 200,000,000 shares authorized, 36,081,615 and 35,831,808 shares issued and outstanding at June 30, 2013 (unaudited) and December 31, 2012, respectively	36	36
Additional paid-in capital	124,363	122,528
Accumulated other comprehensive loss	(82)	(52)
Accumulated deficit	(74,996)	(60,419)
Total stockholders' equity	49,321	62,093
Total liabilities and stockholders' equity	$ 92,517	$ 103,518

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Convertible Notes Payable - Additional Information (Detail) (USD $)				1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jul. 31, 2012	Oct. 31, 2012 October 2012	Oct. 31, 2012 October 2012 Convertible Notes GSK
Debt Instrument [Line Items]
Convertible notes issued					$ 5,400,000
Outstanding principal amount			5,000,000	5,000,000
Accrued interest on convertible notes				400,000
Conversion rate				Oct. 09, 2015
Conversion price per shares	$ 4.00			$ 4.00
Convertible note payable, at fair value	$ 14,592,000	$ 10,134,000

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Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss per Share (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share	4,257,789	16,131,655	3,929,209	16,234,149
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share		13,699,999		13,699,999
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share	2,868,372	2,431,656	2,539,792	2,534,150
Convertible Notes Payable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share	1,389,417		1,389,417

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Summary of Actual Balance Sheet Data and Pro forma Balance Sheet Data to Reflect the Activities Related to Public Offering (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Subsequent Event [Line Items]
Cash, cash equivalents and short-term investments	$ 82,715
Other assets	404	125
Accounts payable and accrued expenses	3,109
Common stock	36	36
Additional paid-in capital	124,363	122,528
Total stockholders' equity	49,321	62,093
Pro Forma
Subsequent Event [Line Items]
Cash, cash equivalents and short-term investments	128,709
Other assets	185
Accounts payable and accrued expenses	2,890
Common stock	41
Additional paid-in capital	170,352
Total stockholders' equity	$ 95,315

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Strategic Alliances and Collaborations - Additional Information (Detail) (USD $)	6 Months Ended		6 Months Ended		6 Months Ended																						1 Months Ended	6 Months Ended											1 Months Ended	6 Months Ended			1 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 GSK	Jun. 30, 2013 GSK Development Commercialization And License Agreement	Jun. 30, 2013 GSK Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK HCV Alliance	Jun. 30, 2013 GSK Preclinical Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Preclinical HCV Alliance	Jun. 30, 2013 GSK Clinical Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Clinical HCV Alliance	Jun. 30, 2013 GSK Regulatory Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Regulatory HCV Alliance	Jun. 30, 2013 GSK Commercialization Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Fourth Micro RNA Targets Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Development Candidate Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Phase One Clinical Trial Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Phase One Clinical Trial HCV Alliance	Jun. 30, 2013 GSK Phase Two Clinical Trial Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Phase Two Clinical Trial HCV Alliance	Jun. 30, 2013 GSK Proof-of-Concept Trial Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Proof-of-Concept Trial HCV Alliance	Jun. 30, 2013 GSK Commercialization Milestones HCV Alliance	Jun. 30, 2013 GSK Minimum Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Minimum HCV Alliance	Jun. 30, 2013 GSK Maximum Immuno-Inflammatory Alliance	Jun. 30, 2013 GSK Maximum HCV Alliance	Oct. 31, 2012 Astra Zeneca	Jun. 30, 2013 Astra Zeneca Agreement	Jun. 30, 2013 Astra Zeneca Preclinical	Jun. 30, 2013 Astra Zeneca Clinical	Jun. 30, 2013 Astra Zeneca Commercialization	Jun. 30, 2013 Astra Zeneca Minimum	Jun. 30, 2013 Astra Zeneca Maximum	Jun. 30, 2013 Biogen Idec	Jun. 30, 2013 Biogen Idec Research Tax Credit Carryforward	Jun. 30, 2013 Biogen Idec Identification Of Micro Rna Biomarker	Jun. 30, 2013 Biogen Idec Validation Of Micro Rna Biomarker	Jun. 30, 2013 Biogen Idec Longitudinal Study Of Patient Samples On Ms Therapy	Jun. 30, 2013 Sanofi	Jun. 30, 2013 Sanofi Preclinical HCV Alliance	Jun. 30, 2013 Sanofi Clinical HCV Alliance	Jun. 30, 2013 Sanofi Regulatory And Commercial Milestones HCV Alliance	Jul. 31, 2013 Sanofi Subsequent Event Development Commercialization And License Agreement	Jul. 31, 2013 Sanofi Subsequent Event Research and Development	Jun. 30, 2013 Sanofi Minimum HCV Alliance	Jun. 30, 2013 Sanofi Maximum HCV Alliance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue recognition period			8 years
Deferred revenue				$ 3,700,000																																								$ 10,100,000
Acceleration of remaining amortization of upfront payment			1,100,000
Revenue through milestone payments					432,500,000	144,500,000	15,500,000	5,500,000	87,000,000	29,000,000	150,000,000	50,000,000	180,000,000	500,000	5,000,000	4,000,000	4,000,000	5,000,000	5,000,000	20,000,000	20,000,000	60,000,000						498,000,000	5,000,000	123,000,000	370,000,000				1,300,000	250,000	500,000	500,000		15,000,000	33,000,000	165,000,000
Royalties based on percentage of net sales																							10.00%	10.00%	20.00%	20.00%						10.00%	20.00%												8.00%	12.00%
Option expiry date																																							2013-12
Option extension period																																							30 days
Initial upfront option payment																												3,000,000						800,000									2,500,000	25,000,000
Upfront payment creditable against future accounts receivables																																											1,250,000
Upfront payment non-creditable portion recognized																																											1,250,000
Number of collaborative areas granted																												3
Collaboration and license agreement date																												August 2012						August 2012
Expected term of research and development plan																											4 years
Common stock issue	36,000	36,000																										25,000,000
Common stock issued	36,081,615	35,831,808																										6,250,000
Price per share																												$ 4.00
Number of days during which AstraZeneca could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock																												365 days
Restricting common stock valuation measurement period																												1 year
Restricting common stock valuation, discounted amount																												4,300,000
Revenue recognition period																																		2 years
Issued promissory note	$ 5,000,000
Conversion to common stock	1,256,232
Conversion rate	$ 4.00

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Related Party Transactions - Additional Information (Detail) (Alnylam, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Alnylam
Related Party Transaction [Line Items]
Service agreement	$ 0.1	$ 0.4

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Related Party Transactions

6 Months Ended

Jun. 30, 2013

Related Party Transactions

8. Related Party Transactions

We have entered into several agreements with related parties in the ordinary course of business to license intellectual property and to procure administrative and research and development support services. These agreements include a Services Agreement with our founding companies Isis Pharmaceuticals, Inc. and Alnylam Pharmaceuticals, Inc. Total costs incurred from services provided from Alnylam under the Services Agreement were $0.1 million and $0.4 million for the three and six months ended June 30, 2013, respectively. Activities under the Services Agreement for the six months ended June 30, 2012 were immaterial.

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Common Stock Reserved for Future Issuance (Detail)	Jun. 30, 2013	Dec. 31, 2012
Equity [Line Items]
Common stock options outstanding	4,647,000	4,720,000
Total common shares reserved for future issuance	8,689,152	7,124,739
Common Stock
Equity [Line Items]
Common stock options outstanding	4,646,541	4,661,549
Common stock shares available for future grant	2,171,920	946,403
Employee Stock Purchase Plan
Equity [Line Items]
Common stock shares available for future grant	481,274	150,000
Convertible Notes Payable
Equity [Line Items]
Total common shares reserved for future issuance	1,389,417	1,366,787

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Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2013

Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss per Share

Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common equivalent shares):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
Convertible preferred stock outstanding		—		13,699,999		—		13,699,999
Common stock options		2,868,372		2,431,656		2,539,792		2,534,150
Convertible note payable		1,389,417		—		1,389,417		—
Total		4,257,789		16,131,655		3,929,209		16,234,149

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Strategic Alliances and Collaborations

6 Months Ended

Jun. 30, 2013

Strategic Alliances and Collaborations

7. Strategic Alliances and Collaborations

The following table summarizes the amounts included in our revenues which resulted from our strategic alliances and collaborations (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
GSK	$	1,303	$	809	$	1,489	$	1,623
Sanofi		2,905		2,500		5,406		5,030
AstraZeneca		465		—		929		—
Biogen Idec		86		—		173		—
Total	$	4,759	$	3,309	$	7,997	$	6,653

GSK

In June 2012, we amended our product development and commercialization agreement with GSK to extend the target selection period for the fourth collaboration target under the agreement. The modification made to the agreement was considered a material modification, which required the application of the new authoritative guidance adopted by us in January 2011 for multiple element arrangements. We determined that the elements within the strategic alliance should be treated as a single unit of accounting because the delivered elements, the opt-in licenses for microRNA product candidates, did not have stand-alone value to GSK. As a result of the extension of the target selection period, we extended the amortization period for the remaining deferred revenue to approximately eight years, which represented our new estimated performance period under the amended agreement. As of June 30, 2013, deferred revenue associated with the product development and commercialization agreement was $3.7 million.

In June 2013, the product development and commercialization agreement with GSK was amended to clarify that RG-101, and other formulations thereof, will be developed by us independently of our alliance with GSK for the treatment of chronic hepatitis C virus (HCV) infection. This amendment removed any further milestone or royalty obligations owed by GSK to us as it relates to RG-101. Concurrent with the amendment, we accelerated the remaining unamortized $1.1 million in deferred revenue associated with the upfront payment from the February 2010 amendment that expanded our agreement with GSK to include potential microRNA therapeutics for the treatment of HCV, which represents that our estimated performance period under the amended agreement is complete.

Immuno-Inflammatory Alliance

The immuno-inflammatory alliance includes contractual milestones. If all the product candidates are successfully developed and commercialized through pre-agreed sales targets we could receive milestone payments up to $432.5 million, including up to $15.5 million for preclinical milestones, up to $87.0 million for clinical milestones, up to $150.0 million for regulatory milestones and up to $180.0 million for commercialization milestones. We are also entitled to receive tiered royalties as a percentage of annual sales which can increase up to the low end of the 10 to 20% range.

We have evaluated the remaining contingent event-based payments under our strategic alliance agreement with GSK and determined that the preclinical and clinical payments meet the definition of a substantive milestone because they are related to events (i) that can be achieved based in whole or in part on our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there was substantive uncertainty at the date the agreement was entered into that the event would be achieved and (iii) that would result in additional payments being due to us. Accordingly, revenue for these achievements will be recognized in its entirety in the period when the milestone is achieved and collectability is reasonably assured. Other contingent event-based payments under the strategic alliance agreement for which payment is contingent upon the results of GSK’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured. We can earn the following preclinical milestones: $0.5 million upon the selection of a fourth microRNA target and $5.0 million upon the selection of a development candidate for each of the selected three targets. We can also earn the following clinical milestones for each of the selected three targets: $4.0 million for the initiation of a Phase 1 clinical trial; $5.0 million for the initiation of a Phase 2 clinical trial; and $20.0 million if GSK chooses to opt-in to the program following the completion of a proof-of-concept trial.

Hepatitis C Virus (“HCV”) Alliance

Notwithstanding the foregoing, GSK has retained its interest in the miR-122 program in HCV, and miR-122 remains a collaboration target under the alliance. If the HCV program is successful, we could receive contractual milestone payments up to $144.5 million, including up to $5.5 million for preclinical milestones, up to $29.0 million for clinical milestones, up to $50.0 million for regulatory milestones and up to $60.0 million for commercialization milestones. In addition, we will receive tiered royalties which can increase up to the low end of the 10 to 20% range on sales from any product that GSK successfully commercializes under this alliance.

We have evaluated the remaining contingent event-based payments under our strategic alliance agreement with GSK and determined that the preclinical and clinical payments meet the definition of a substantive milestone because they are related to events (i) that can be achieved based in whole or in part on our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there was substantive uncertainty at the date the agreement was entered into that the event would be achieved and (iii) that would result in additional payments being due to us. Accordingly, revenue for these achievements will be recognized in its entirety in the period when the milestone is achieved and collectability is reasonably assured. Other contingent event-based payments under the strategic alliance agreement for which payment is contingent upon the results of GSK’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured. We can earn a preclinical milestone of $5.5 million upon the selection of a development candidate. We can also earn the following clinical milestones: $4.0 million for initiation of a Phase 1 clinical trial; $5.0 million for the initiation of a Phase 2 clinical trial; and $20.0 million if GSK chooses to opt-in to the program following the completion of a proof-of-concept trial.

Sanofi-Aventis

In July 2012, we amended and restated our collaboration and license agreement with Sanofi to expand the potential therapeutic applications of the microRNA alliance targets to be developed under such agreement. The modification made to the agreement was considered a material modification, which required the application of the new authoritative guidance adopted by us in January 2011 for multiple element arrangements. We determined that the elements within the strategic alliance agreement with Sanofi should be treated as a single unit of accounting because the delivered elements did not have stand-alone value to Sanofi. The following elements were delivered as part of the strategic alliance with Sanofi: (1) a license for up to four microRNA targets; and (2) a research license under our technology alliance.

In June 2013, the original research term expired, upon which we and Sanofi entered into an option agreement pursuant to which we granted Sanofi an exclusive right to negotiate the co-development and commercialization of certain of our unencumbered microRNA programs and we were granted the exclusive right to negotiate with Sanofi for co-development and commercialization of certain miR-21 anti-miRs in oncology and Alport’s Syndrome. These options expire in December 2013, subject to a 30-day extension in certain circumstances. In July 2013, we received an upfront payment of $2.5 million, of which $1.25 million is creditable against future amounts payable by Sanofi to us under any future co-development and commercialization agreement we enter pursuant to the option agreement. The non-creditable portion of this payment, $1.25 million, will be recognized as revenue over the option period from the effective date of the option agreement in June 2013 until the expiration of the option period in December 2013. Revenue associated with the creditable portion of this option payment will be deferred until the option period expires, or is re-evaluated in connection with a creditable transaction. In addition, we agreed to continue specified research on the miR-21 programs during the option period. As a result of the expiration of the research term and subsequent option agreement, we re-evaluated our estimated period of performance and are now amortizing the remaining deferred revenue of $10.1 million associated with the initial upfront payment of $25.0 million ratably until the expiration of the option period in December 2013.

We have evaluated the remaining contingent event-based payments under our strategic alliance agreement with Sanofi and determined that the preclinical payments meet the definition of a substantive milestone because they are related to events (i) that can be achieved based in whole or in part on our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there was substantive uncertainty at the date the agreement was entered into that the event would be achieved and (iii) that would result in additional payments being due to us. Accordingly, revenue for these achievements will be recognized in its entirety in the period when the milestone is achieved and collectability is reasonably assured. Other contingent event-based payments under the strategic alliance agreement for which payment is contingent upon the results of Sanofi’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured. As of the expiration of the research term in June 2013, Sanofi no longer has rights to select additional microRNA targets. As such, we can earn a preclinical milestone of $15.0 million upon the filing of an Investigational New Drug application (“IND”) for the selected microRNA target. We can also receive milestone payments of up to $33.0 million for clinical milestones and up to $165.0 million for regulatory and commercial milestones. In addition, we are entitled to receive royalties based on a percentage of net sales which will range from 8 to 12% range, depending upon the volume of sales.

AstraZeneca

In August 2012, we entered into a collaboration and license agreement with AstraZeneca. Under the terms of the agreement, we have agreed to collaborate with AstraZeneca to identify, research and develop compounds targeting three microRNA alliance targets primarily in the fields of cardiovascular diseases, metabolic diseases and oncology and granted to AstraZeneca an exclusive, worldwide license to thereafter develop, manufacture and commercialize lead compounds designated by AstraZeneca in the course of the collaboration activities against the alliance targets for all human therapeutic uses. Under the terms of the agreement we are required to use commercially reasonable efforts to perform all research, development and manufacturing activities described in the research plan, at our cost, until the acceptance of an IND or the end of the research term, which extends until the fourth anniversary of the date of the agreement, and may be extended only by mutual written agreement of us and AstraZeneca. Following the earlier to occur of the acceptance of an IND in a major market or the end of the research term, AstraZeneca will assume all costs, responsibilities and obligations for further development, manufacture and commercialization of alliance product candidates.

Under the terms of the agreement, we received an upfront payment of $3.0 million in October 2012. We determined the elements within the strategic alliance agreement should be treated as a single unit of accounting because the delivered element, the license, does not have stand-alone value. As a result, we are recognizing revenue related to the upfront payment on a straight-line basis over our estimated period of performance, which is four years based on the expected term of the research and development plan. If all three targets are successfully developed and commercialized through pre-agreed sales targets we could receive milestone payments up to $498.0 million, including up to $5.0 million for preclinical milestones, up to $123.0 million for clinical milestones, and up to $370.0 million for commercialization milestones. In addition, we are entitled to receive royalties based on a percentage of net sales which will range from the mid-single digits to the low end of the 10 to 20% range, depending upon the product and the volume of sales, which royalties may be reduced in certain, limited circumstances.

We have evaluated the contingent event-based payments under our strategic alliance agreement with AstraZeneca and determined that the preclinical payments meet the definition of substantive milestones because they are related to events (i) that can be achieved based in whole or in part on our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there was substantive uncertainty at the date the agreement was entered into that the event would be achieved and (iii) that would result in additional payments being due to us. Accordingly, revenue for these achievements will be recognized in its entirety in the period when the milestone is achieved and collectability is reasonably assured. Other contingent event-based payments under the strategic alliance agreement for which payment is contingent upon the results of AstraZeneca’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured.

Concurrently with the collaboration and license agreement, we entered into a Common Stock Purchase Agreement (“CSPA”) with AstraZeneca, pursuant to which we agreed to sell to AstraZeneca an aggregate of $25.0 million of our common stock in a private placement concurrently with our initial public offering, at a price per share equal to the initial public offering price. In October 2012, in accordance with the CSPA, we sold AstraZeneca 6,250,000 shares of our common stock at a price per share of $4.00. Further, the CSPA stipulated that AstraZeneca could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock for a 365-day period following the effective date of our initial public offering. Accounting guidance for multiple element arrangements contains a presumption that separate contracts negotiated and/or entered into at or near the same time with the same entity were negotiated as a package and should be evaluated as a single agreement. In order to quantify the discount applied to the shares of common stock due to the lack of marketability, we had an independent valuation performed to measure the value of restricting common stock for a period of one year. Based upon restricted stock studies of similar duration and a Black-Scholes valuation to measure the lack of marketability discount, $4.3 million was attributed to the collaboration and license agreement. We continue to recognize the $4.3 million into revenue ratably over the estimated period of performance of the collaboration.

Biogen Idec

In August 2012, we entered into a collaboration and license agreement with Biogen Idec pursuant to which we and Biogen Idec have agreed to collaborate on microRNA biomarkers for multiple sclerosis (“MS”). Under the terms of the agreement, we granted Biogen Idec an exclusive, royalty free, worldwide license to our interest in the collaboration intellectual property for the purpose of commercializing non-microRNA products for the treatment, diagnosis and prevention of MS and non-MS diseases and disorders. We also granted Biogen Idec an exclusive, royalty-free, worldwide license, with the right to sublicense, to our interest in the collaboration intellectual property (and a non-exclusive license to our background intellectual property) for the purpose of commercializing products for the diagnosis of MS. We also granted Biogen Idec a right of first negotiation on certain commercial transactions relating to microRNA products which utilize intellectual property developed during the collaboration. Pursuant to the terms of the agreement, in August 2012 we received an upfront payment of $0.8 million. We are also eligible to receive research milestone payments up to an aggregate of $1.3 million. We considered the elements within the collaboration and license agreement as a single unit of accounting because the delivered element, the license, does not have stand-alone value. As a result, we are recognizing revenue relating to the upfront payment of $0.8 million on a straight-line basis over our estimated period of performance, which is approximately two years based on the expected term of the research and development plan.

In June 2013, we amended the collaboration and license agreement with Biogen Idec to provide for revised terms with respect to phase 1 of the research plan under the Biogen agreement and revised payment provisions relating to the phase 1 milestones. The Biogen amendment does not modify the maximum dollar amount we were originally eligible to receive in connection with phase 1 milestones under the Biogen agreement, or our estimated period of performance.

We have evaluated the contingent event-based payments under our collaboration and license agreement with Biogen Idec and determined that the research payments meet the definition of substantive milestones because they are related to events (i) that can be achieved based in whole or in part on our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there was substantive uncertainty at the date the agreement was entered into that the event would be achieved and (iii) that would result in additional payments being due to us. Accordingly, revenue for these achievements will be recognized in its entirety in the period when the milestone is achieved and collectability is reasonably assured. We can earn the following research milestones: $0.25 million for identification of a microRNA biomarker; $0.5 million for validation of the microRNA biomarker in a second independent sample set; and $0.5 million upon the refinement of the microRNA biomarker signature from a longitudinal study of patient samples on MS therapy.

Concurrently with the collaboration and license agreement, we entered into a note purchase agreement with Biogen Idec, pursuant to which we issued Biogen Idec a convertible promissory note in the principal amount of $5.0 million. The $5.0 million note plus accrued interest converted into 1,256,232 shares of our common stock upon the closing of our initial public offering in October 2012 at a conversion price of $4.00 per share.

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Net Loss Per Share

6 Months Ended

Jun. 30, 2013

Net Loss Per Share

2. Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of convertible preferred stock, options outstanding under our stock option plan and convertible notes payable. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position.

Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common equivalent shares):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
Convertible preferred stock outstanding		—		13,699,999		—		13,699,999
Common stock options		2,868,372		2,431,656		2,539,792		2,534,150
Convertible note payable		1,389,417		—		1,389,417		—
Total		4,257,789		16,131,655		3,929,209		16,234,149

In October 2012, all convertible preferred stock converted into common stock in conjunction with our initial public offering. As of June 30, 2013 we had a convertible note payable outstanding with a principal balance of $5.4 million that was convertible into common shares at $4.00 per share, at the option of the note holder.

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Stock-Based Compensation (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 759	$ 205	$ 1,546	$ 318
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	571	121	1,070	183
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 188	$ 84	$ 476	$ 135

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Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2013

Common Stock Reserved for Future Issuance

The following shares of common stock are reserved for future issuance at June 30, 2013:

	June 30, 2013		December 31, 2012
Common stock options outstanding		4,646,541		4,661,549
Common stock options available for future grant		2,171,920		946,403
Employee Stock Purchase Plan		481,274		150,000
Convertible note payable		1,389,417		1,366,787
Total common shares reserved for future issuance		8,689,152		7,124,739

Stock Option Activity

The following table summarizes our stock option activity under all stock option plans for the six months ended June 30, 2013 (in thousands):

	Number of options			Weighted average exercise price
Options outstanding at December 31, 2012		4,720		$	2.11
Granted		340		$	7.81
Exercised		(223	)	$	0.91
Canceled/forfeited/expired		(190	)	$	3.33
Options outstanding at June 30, 2013		4,647		$	2.53

Assumptions Used to Estimate Fair Value of Stock Options Using Black-Scholes Option Pricing Model

The following table summarizes the weighted average assumptions we used in our Black-Scholes calculations:

	Three months ended June 30,						Six months ended June 30,
	2013			2012			2013			2012
Employee Stock Options:
Risk-free interest rate		1.5	%		1.0	%		1.4	%		1.1	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected volatility		67.6	%		70.8	%		67.5	%		71.0	%
Expected term (years)		6.1			6.1			6.1			6.1

Stock-Based Compensation

The following table summarizes the allocation of our stock compensation expense (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
Research and development	$	571	$	121	$	1,070	$	183
General and administrative		188		84		476		135
Total	$	759	$	205	$	1,546	$	318

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Basis of Presentation and Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2013

Basis of Presentation

Basis of Presentation

The accompanying unaudited condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In management’s opinion, the accompanying financial statements reflect all adjustments, consisting of normal recurring adjustments, considered necessary for a fair presentation of the results for the interim periods presented.

Interim financial results are not necessarily indicative of results anticipated for the full year. These unaudited condensed financial statements should be read in conjunction with our audited financial statements and footnotes included in our Annual Report on Form 10-K for the year ended December 31, 2012, from which the balance sheet information herein for the year ended December 31, 2012 was derived.

On September 7, 2012, our board of directors approved a one-for-two reverse stock split of our common stock. The accompanying condensed financial statements and notes to the condensed financial statements give retroactive effect to the reverse split for all periods presented. No further splits (or reverse splits) of our common stock have been contemplated as of June 30, 2013.

Use of Estimates

Use of Estimates

Our financial statements are prepared in accordance with GAAP. The preparation of our financial statements requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions.

Revenue Recognition

Revenue Recognition

As of June 30, 2013, we have ongoing strategic alliance agreements and collaborations with GlaxoSmithKline plc (“GSK”), Sanofi, AstraZeneca AB (“AstraZeneca”) and Biogen Idec MA Inc (“Biogen Idec”). Our revenues generally consist of upfront payments for licenses or options to obtain licenses in the future, research and development funding and milestone payments, as well as funding received under government grants. We recognize revenues when all four of the following criteria are met: (1) persuasive evidence that an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured.

For these multiple element arrangements, deliverables under our agreements are accounted for as separate units of accounting provided that (i) a delivered item has value to the customer on a stand-alone basis; and (ii) if the agreement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the control of the vendor. The allocation of consideration amongst the units of accounting under our strategic alliance agreements and collaborations are derived using a “best estimate of selling price” if vendor specific objective evidence and third-party evidence of fair value is not available.

Milestones

We recognize revenue from milestone payments when earned, provided that (i) the milestone event is substantive in that it can only be achieved based in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance and its achievability was not reasonably assured at the inception of the agreement, (ii) we do not have ongoing performance obligations related to the achievement of the milestone and (iii) it would result in the receipt of additional payments. A milestone payment is considered substantive if all of the following conditions are met: (i) the milestone payment is non-refundable; (ii) achievement of the milestone was not reasonably assured at the inception of the arrangement; (iii) substantive effort is involved to achieve the milestone; and (iv) the amount of the milestone payments appears reasonable in relation to the effort expended, the other milestones in the arrangement and the related risk associated with the achievement of the milestone. Any amounts received under the agreements in advance of performance, if deemed substantive, are recorded as deferred revenue and recognized as revenue as we complete our performance obligations.

Generally, the milestone events contained in our strategic alliance agreements and collaborations coincide with the progression of our product candidates from target selection, to clinical candidate selection, to clinical trial, to regulatory approval and then to commercialization. The process of successfully discovering a new development candidate, having it approved and ultimately sold for a profit is highly uncertain. As such, the milestone payments we may earn from our partners involve a significant degree of risk to achieve. Therefore, as a product candidate progresses through the stages of its life-cycle, the value of the product candidate generally increases.

Deferred Revenue

Deferred Revenue

Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying condensed balance sheets. Amounts not expected to be recognized within the next 12 months are classified as non-current deferred revenue.

Stock-Based Compensation

Stock-Based Compensation

We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period.

We account for stock options granted to non-employees other than members of our board of directors, which primarily consist of members of our scientific advisory board, using the fair value approach. Stock options granted to non-employees are subject to periodic revaluation over their vesting terms.

Fair Value Option

Fair Value Option

Applicable accounting policies permit entities to choose, at specified election dates, to measure specified items at fair value if the decision about the election is: 1) applied instrument by instrument, 2) irrevocable, and 3) applied to an entire instrument.

In July 2012, we amended and restated our $5.0 million convertible promissory note originally issued in February 2010 to GSK (“2010 GSK Note”), which was accounted for as a debt extinguishment of the original note. We elected to measure the 2010 GSK Note under the fair value option. Upon initial measurement, the difference between the carrying value of the original note and the fair value of the 2010 GSK Note was recorded as a loss on extinguishment of debt to non-operating earnings. Thereafter, any change to the fair value of the 2010 GSK Note is recorded as a gain (loss) from valuation of convertible note payable to non-operating earnings.

Recent Accounting Pronouncements

Recent Accounting Pronouncements

In February 2013, the FASB issued Accounting Standards Update No. 2013-02, Comprehensive Income (Topic 220): Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income (“ASU 2013-02”). This update will require companies to present information about amounts reclassified out of accumulated other comprehensive income and their corresponding effect on net income in one place and reference the amounts to the related footnote disclosures. Current accounting standards present this information in different places throughout the financial statements. ASU 2013-02 was effective for us for the six months ended June 30, 2013. The adoption of ASU 2013-02 had no impact on our financial condition, results of operations, or cash flows.

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Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Jul. 31, 2012
Organization And Summary Of Significant Accounting Policies [Line Items]
Outstanding principal amount	$ 5.0

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Strategic Alliances and Collaborations (Tables)

6 Months Ended

Jun. 30, 2013

Revenue from Strategic Alliance

The following table summarizes the amounts included in our revenues which resulted from our strategic alliances and collaborations (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
GSK	$	1,303	$	809	$	1,489	$	1,623
Sanofi		2,905		2,500		5,406		5,030
AstraZeneca		465		—		929		—
Biogen Idec		86		—		173		—
Total	$	4,759	$	3,309	$	7,997	$	6,653

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Document and Entity Information	6 Months Ended
	Jun. 30, 2013	Aug. 12, 2013
Document Information [Line Items]
Document Type	10-Q/A
Amendment Flag	true
AmendmentDescription	This Amendment No. 1 on Form 10-Q/A (this “Amendment”) amends the Quarterly Report on Form 10-Q of Regulus Therapeutics Inc. for the quarter ended June 30, 2013, originally filed with the Securities and Exchange Commission on August 14, 2013 (the “Original Filing”). We are filing this Amendment solely to correct a clerical error on the facing page of the Original Filing which stated that we had 36,155,745 shares of Common Stock outstanding as of August 12, 2013. The correct number of shares of Common Stock outstanding as of August 12, 2013 is 41,330,745 shares, as reflected on the facing page of this Amendment. In connection with the filing of this Amendment and pursuant to the rules of the Securities and Exchange Commission, we are including with this Amendment new certifications by our principal executive and principal financial officer. Except as described above, no other changes have been made to the Original Filing. The Original Filing continues to speak as of the date of the Original Filing, and we have not updated the disclosures contained therein to reflect any events which occurred at a date subsequent to the filing of the Original Filing other than as expressly indicated in this Amendment.
Document Period End Date	Jun. 30, 2013
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q2
Trading Symbol	RGLS
Entity Registrant Name	REGULUS THERAPEUTICS INC.
Entity Central Index Key	0001505512
Current Fiscal Year End Date	--12-31
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		41,330,745

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Subsequent Events (Tables)

6 Months Ended

Jun. 30, 2013

Pro Forma Net Proceeds from Initial Public Offering

Pro forma net proceeds from our public offering were determined as follows (in thousands):

Gross proceeds (including over-allotment)	$	49,163
Underwriting discounts and commissions		(2,950	)
Estimated total offering costs		(360	)
Offering costs paid as of June 30, 2013		—
Pro forma net proceeds	$	45,853

Summary of Actual Balance Sheet Data and Pro forma Balance Sheet Data to Reflect the Activities Related to Public Offering

The following table summarizes certain actual balance sheet data and pro forma balance sheet data to reflect the activities related to our public offering noted above, as of June 30, 2013 (in thousands):

	June 30, 2013		Pro forma June 30, 2013
Cash, cash equivalents and short-term investments	$	82,715	$	128,709
Other assets		404		185
Accounts payable and accrued expenses		3,109		2,890
Common stock		36		41
Additional paid-in capital		124,363		170,352
Total stockholders’ equity	$	49,321	$	95,315