10. Convertible preferred stock and stockholders’ equity

As described in Note 1, all previously outstanding convertible preferred stock was converted to common stock upon the completion of the Company’s IPO in June 2018.

Series A preferred stock

On May 4, 2016, the Company entered into a purchase agreement (the “Series A Purchase Agreement”) for a private placement of up to 151,772,701 shares of Series A‑1 Convertible Preferred Stock (the “Series A‑1 Preferred Stock”) and 173,453,018 shares of Series A‑2 Convertible Preferred Stock (the “Series A‑2 Preferred Stock” and together with the Series A‑1 Preferred Stock, the “Series A Preferred Stock”) under the Series A Purchase Agreement. Of the 325,225,719 authorized shares of Series A Preferred Stock, 151,772,701 shares were designated Series A‑1 Preferred Stock at $0.16472 per share and 173,453,018 shares were designated Series A‑2 Preferred Stock at $0.23061 per share. The Series A Purchase Agreement obligated the investors to purchase, at the election of the Company’s board of directors (the “Board of Directors”), the Series A‑2 Preferred Stock at $0.23061 per share upon achieving certain clinical milestones. The determination as to whether the milestone event was met was subject to the certification by (i) the Board of Directors and (ii) the holders of at least a majority of the then‑outstanding Series A Preferred Stock. The Series A Purchase Agreement also obligated the Company to sell to each investor that elects to purchase such investor’s portion of Series A‑2 Preferred Stock at any time such number of shares of Series A‑2 Preferred Stock pro rata in accordance with the number of Series A‑1 Preferred Stock purchased at the initial closing (the “Tranche Rights”). In January 2017, the Company achieved its clinical milestone obligating the Company to sell its Series A‑2 Preferred Stock at $0.23061 per share.

Series B preferred stock

On December 11, 2017, the Company entered into a purchase agreement (the “Series B Purchase Agreement”) for a private placement of 234,954,520 shares of Series B Convertible Preferred Stock (the “Series B Preferred Stock”) in exchange for $69.8 million, net of issuance costs. All 234,954,520 authorized shares of Series B Preferred Stock were designated at $0.29793 per share.

Dividends

The holders of the Series A and Series B Preferred Stock are not entitled to receive dividends unless declared by the Board of Directors of the Company in accordance with the Company’s certificate of incorporation, as amended from time to time. No dividends have been declared since inception.

Liquidation preference

Upon a liquidation of the Company, the assets of the Company or proceeds available for distribution to the Company’s stockholders shall be paid as follows: (a) first, for each share of Series B Preferred Stock the greater of (i) one times the original purchase price plus declared and unpaid dividends on such share, or (ii) such amount as would have been payable had all shares of Series B Preferred Stock been converted to common stock immediately prior to such liquidation; and (b) then, for each share of Series A Preferred Stock the greater of (i) one times the original purchase price plus declared and unpaid dividends on such share, or (ii) such amount as would have been payable had all shares of (A) Series A‑1 Preferred Stock been converted to common stock, in the case of Series A‑1 Preferred Stock and (B) Series A‑2 Preferred Stock been converted to common stock, in the case of Series A‑2 Preferred Stock immediately prior to such liquidation. The balance of any proceeds shall be distributed pro rata to holders of common stock. If upon liquidation, the assets of the Company or proceeds available for distribution to its stockholders are insufficient to pay the holders of the Series B Preferred Stock as to their respective liquidation preferences the full amount entitled, the holders of the Series B Preferred Stock shall share ratably any distribution of the assets or proceeds available for distribution. If upon liquidation, the assets of the Company or proceeds available for distribution to its stockholders, after payment in full of the liquidation preferences to holders of the Series B Preferred Stock, are insufficient to pay the holders of the Series A Preferred Stock as to their respective liquidation preferences the full amount entitled, the holders of the Series A Preferred Stock shall share ratably any distribution of the remaining assets or proceeds available for distribution.

Conversion update

Shares of preferred stock are convertible into such number of fully paid and non‑assessable shares of common stock as determined by dividing the original issuance price by the conversion price at the time in effect, subject to adjustment. The original conversion price is $0.16472 for Series A‑1 Preferred Stock, $0.23061 for Series A‑2 Preferred Stock, and $0.29793 for Series B Preferred Stock, in each case, subject to adjustments to reflect the issuance of common stock, options, warrants, or other rights to subscribe for or to purchase common stock for a consideration per share, less than the conversion price then in effect and subsequent stock dividends, stock splits, combinations, or recapitalizations.

Conversion is at the option of the respective holders of Series A and Series B Preferred Stock, although conversion is automatic upon the earlier of (a) the consummation of an underwritten public offering resulting in gross proceeds to the Company of at least $70 million and a share price of at least $0.446895 per share (subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization), or (b) the date and time, or the occurrence of an event, specified by the vote or written consent of the holders of at least majority of the then outstanding shares of Series A and Series B Preferred Stock, respectively; provided, that the outstanding shares of Series B Preferred Stock will not convert into shares of common stock pursuant to clause (b) without the approval of at least a majority of the then‑outstanding Series B Preferred Stock, including one or more holders of Series B Preferred Stock that (i) individually, or together with such holders’ affiliates, do not hold any Series A Preferred Stock and (ii) individually or collectively are the record holders of at least 30,208,439 shares of Series B Preferred Stock, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization with respect to the Series B Preferred Stock.

Voting rights

Holders of the Series A and Series B Preferred Stock are entitled to vote as a single class with the holders of common stock and shall have one vote for each equivalent common share into which the preferred stock is convertible. The holders of Series B Preferred Stock are entitled to elect one director to the Board of Directors; the holders of Series A Preferred Stock are entitled to elect four directors to the Board of Directors; and the holders of Preferred Stock and the holders of common stock on an as‑converted to common stock basis, are entitled to elect the remaining directors.

Common stock

As of December 31, 2019 and 2018, the Company had reserved common stock for issuance as follows (in thousands):


	As of December 31,
	2019	2018
Stock options issued and outstanding	4,798	3,959
Unvested restricted stock	1,032	195
	5,830	4,154