1. Nature of the Business Proteostasis Therapeutics, Inc. (the Company) was incorporated in Delaware on December 13, 2006. The Company is a clinical stage biopharmaceutical company committed to the discovery and development of novel therapeutics to treat cystic fibrosis (CF) and other diseases caused by an imbalance in the proteostasis network, a set of pathways that control protein biosynthesis, folding, trafficking and clearance. The Company focuses on identifying therapies that restore protein function. CF is a disease caused by defects in the function or abundance of cystic fibrosis transmembrane conductance regulator (CFTR). The Companys CF focused pipeline consists of novel CFTR modulators including correctors, potentiators and amplifiers. Upon discovery of amplifiers, a novel class of CFTR modulators, the Company has exploited its novel mechanism of action as a drug screening tool and has subsequently identified correctors and potentiators to be developed as part of combination therapies. Investigational agents representative of all three classes of CFTR modulators are currently in clinical development and include PTI-801, a third generation CFTR corrector, PTI-808, a CFTR potentiator, and PTI-428, a CFTR amplifier. The Company is pursuing proprietary dual combination of PTI-801 and PTI-808, and proprietary triple combination of PTI-801, PTI-808 and PTI-428 as product opportunities. The Company is developing and, if approved, intends to commercialize their own proprietary combination therapies for CF patients who have at least one F508del mutation, representing the majority of the patient population. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to secure additional capital to fund operations. 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 and infrastructure and extensive compliance-reporting capabilities. Even if the Companys product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In accordance with ASC 205-40, Going Concern (ASC 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Companys ability to continue as a going concern within one year after the date the financial statements are issued . The Company has incurred losses and negative cash flows from operations since its inception. As of September 30, 2019, the Company had an accumulated deficit of $324.8 million. During the nine months ended September 30, 2019, the Company incurred losses of $47.3 million and used $41.6 million of cash in operations. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. The Company expects that its cash, cash equivalents and short-term investments of $77.8 million will be sufficient to fund its operating expenses and capital requirements, based upon its current operating plan, for at least 12 months from the date that these financial statements are issued. The Company will seek additional funding through public financings, debt financings, collaboration agreements, strategic alliances and licensing arrangements. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all, and the Company may not be able to enter into collaborations or other arrangements. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. In May 2019, the Company entered into a sales agreement with H.C. Wainwright &amp; Co., LLC (HCW) with respect to an at-the-market (ATM) offering program under which the Company may issue and sell, from time to time at its sole discretion, shares of its common stock, in an aggregate offering amount of up to $56.6 million (the HCW ATM program). HCW acts as the Companys sales agent and will use commercially reasonable efforts to sell shares of common stock from time to time, based upon instruction from the Company. Common stock will be sold at prevailing market prices at the time of the sale; and as a result, prices may vary. The Company will pay HCW up to 3% of the gross proceeds from any common stock sold through the sales agreement. As of September 30, 2019, the Company has not sold any shares of its common stock under the HCW ATM program. In the fourth quarter of 2018, the Company completed a follow-on public offering whereby the Company sold 12,650,000 shares of common stock at a public offering price of $6.75 per share, which included 1,650,000 shares issued pursuant to the exercise by the underwriters of their option to purchase additional shares of common stock. The Company received net proceeds of $80.1 million, after deducting underwriting discounts and commissions and other offering expenses. In March 2018, the Company entered into a sales agreement with Leerink Partners LLC (Leerink) with respect to an ATM offering program under which the Company could have issued and sold, from time to time at its sole discretion, shares of its common stock, in an aggregate offering amount of up to $50.0 million. As of March 31, 2019, the Company had sold an aggregate of 3,475,166 shares of its common stock for total net proceeds of approximately $21.6 million under the ATM program. On March 28, 2019 the Company delivered a notice of termination of the ATM offering program to Leerink, effective as of April 7, 2019.