Nature of the Business and Basis of Presentation	12 Months Ended
Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined rare diseases in areas of high unmet medical need. The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing, and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Sales of Common Stock On July 22, 2019, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $63.9 million after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock. On July 5, 2019, in connection with the IPO, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto for periods prior to the reverse stock split have been retroactively adjusted to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. On June 9, 2020, the Company issued and sold 4,029,411 shares of common stock to investors in a private placement at a price of $17.00 per share, resulting in net proceeds of $64.3 million after deducting offering costs. On August 11, 2020, the Company entered into an Equity Distribution Agreement with Piper Sandler & Co. (“Piper Sandler”), as sales agent, pursuant to which the Company may offer and sell shares of its common stock with an aggregate offering price of up to $75.0 million under an “at-the-market” offering program (the “ATM Offering”). The Equity Distribution Agreement provides that Piper Sandler will be entitled to a sales commission equal to 3.0% of the gross sales price per share of all shares sold under the ATM Offering. From the initiation of the ATM Offering through December 31, 2020, the Company has issued and sold 550,000 shares under the ATM Offering, resulting in aggregate net proceeds of $5.7 million after deducting issuance costs of $0.2 million. On January 22, 2021, the Company completed a public offering of its common stock and issued and sold 4,600,000 shares of common stock at a public offering price of $11.00 per share, resulting in net proceeds of $46.4 million after deducting underwriting discounts and commissions and estimated offering expenses. Liquidity The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the sale of shares of common stock in public offerings, a private placement, and the ATM Offering, through issuances of convertible preferred stock, and from upfront payments received from the collaboration and license agreements with Acceleron Pharma Inc. (“Acceleron”) and MyoKardia, Inc. (“MyoKardia”), a wholly-owned subsidiary of Bristol Myers Squibb Company. As of December 31, 2020, the Company had an accumulated deficit of $221.6 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts. The Company expects to finance its future cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. As of the date of issuance of these financial statements, the Company expects that its cash, cash equivalents, and marketable securities, together with the net proceeds from the sale of its common stock in a public offering on January 22, 2021 of $46.4 million, will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of issuance of these financial statements. However, the Company has based this estimate on assumptions that may prove to be wrong, and its operating plan may change as a result of many factors currently unknown to it. As a result, the Company could deplete its capital resources sooner than it currently expects. If the Company is unable to raise additional funds through equity or debt financings when needed, it may be required to delay, limit, reduce or terminate development or future commercialization efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself.

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the Business and Basis of Presentation

Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined rare diseases in areas of high unmet medical need.

The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing, and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Sales of Common Stock

On July 22, 2019, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $63.9 million after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock.

On July 5, 2019, in connection with the IPO, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto for periods prior to the reverse stock split have been retroactively adjusted to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital.

On June 9, 2020, the Company issued and sold 4,029,411 shares of common stock to investors in a private placement at a price of $17.00 per share, resulting in net proceeds of $64.3 million after deducting offering costs.

On August 11, 2020, the Company entered into an Equity Distribution Agreement with Piper Sandler & Co. (“Piper Sandler”), as sales agent, pursuant to which the Company may offer and sell shares of its common stock with an aggregate offering price of up to $75.0 million under an “at-the-market” offering program (the “ATM Offering”). The Equity Distribution Agreement provides that Piper Sandler will be entitled to a sales commission equal to 3.0% of the gross sales price per share of all shares sold under the ATM Offering. From the initiation of the ATM Offering through December 31, 2020, the Company has issued and sold 550,000 shares under the ATM Offering, resulting in aggregate net proceeds of $5.7 million after deducting issuance costs of $0.2 million.

On January 22, 2021, the Company completed a public offering of its common stock and issued and sold 4,600,000 shares of common stock at a public offering price of $11.00 per share, resulting in net proceeds of $46.4 million after deducting underwriting discounts and commissions and estimated offering expenses.

Liquidity

The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the sale of shares of common stock in public offerings, a private placement, and the ATM Offering, through issuances of convertible preferred stock, and from upfront payments received from the collaboration and license agreements with Acceleron Pharma Inc. (“Acceleron”) and MyoKardia, Inc. (“MyoKardia”), a wholly-owned subsidiary of Bristol Myers Squibb Company. As of December 31, 2020, the Company had an accumulated deficit of $221.6 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts. The Company expects to finance its future cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements.

As of the date of issuance of these financial statements, the Company expects that its cash, cash equivalents, and marketable securities, together with the net proceeds from the sale of its common stock in a public offering on January 22, 2021 of $46.4 million, will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of issuance of these financial statements. However, the Company has based this estimate on assumptions that may prove to be wrong, and its operating plan may change as a result of many factors currently unknown to it. As a result, the Company could deplete its capital resources sooner than it currently expects. If the Company is unable to raise additional funds through equity or debt financings when needed, it may be required to delay, limit, reduce or terminate development or future commercialization efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself.