Amendment: SEC Form SCHEDULE 13D/A filed by Zymeworks Inc.

357 Tehama Street #3, San Francisco, CA 94103

EcoR1 is an investment adviser to investment funds, including Qualified Fund (collectively, the "Funds"). EcoR1 is the general partner of the Funds. Mr. Nodelman is the manager and control person of EcoR1.

During the last five years, none of the reporting persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

During the last five years, none of the reporting persons was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

For citizenship of the reporting persons, see Item 6 of each reporting person's cover page.

The Funds used their working capital to purchase the Issuer's Common Stock. EcoR1 Capital Fund Qualified, L.P. paid $192,880,935.01 in the aggregate for its Common Stock, which includes $47,364,999.62 paid by the fund to acquire prefunded warrants that were exercised by the fund on June 26, 2025. EcoR1 Capital Fund, L.P. paid $12,853,702.36 in the aggregate for its Common Stock, which includes $2,634,993.17 paid by the fund to acquire prefunded warrants that were exercised by the fund on June 26, 2025.

Scott Platshon, one of EcoR1's employees, resigned from the Issuer's board of directors as of November 16, 2025. Concurrently with his resignation, Mr. Platshon was appointed as the Issuer's Acting Chief Investment Officer. Oleg Nodelman, the manager of EcoR1, is a member of the Issuer's board of directors. The reporting persons acquired the Stock for investment purposes based on their belief that the Issuer's Common Stock, when purchased, was undervalued and represented an attractive investment opportunity. The reporting persons will routinely monitor the Issuer regarding a wide variety of factors that affect their investment considerations, including, current and anticipated future trading prices of the Common Stock and other securities, the Issuer's operations, assets, prospects, financial position, and business development, Issuer's management, Issuer-related competitive and strategic matters, general economic, financial market and industry conditions, and other investment considerations. Depending on their evaluation of various factors, the reporting persons may take such actions regarding their holdings of the Issuer's securities as they deem appropriate in light of circumstances existing from time to time. Such actions may include purchasing additional Common Stock in the open market, through privately negotiated transactions with third parties or otherwise, and selling at any time, in the open market, through privately negotiated transactions with third parties or otherwise, all or part of the Common Stock that they now own or hereafter acquire. The reporting persons also may from time to time enter into or unwind hedging or other derivative transactions with respect to the Common Stock or pledge their interests in the Common Stock to obtain liquidity. In addition, from time to time the reporting persons and their representatives and advisers may communicate with other stockholders, industry participants and other interested parties about the Issuer. The reporting persons have no present plan or proposal that relates to or would result in any of the actions specified in clauses (a) through (j) of Item 4 of Schedule 13D. However, the reporting persons may recommend action to the Issuer's management, board of directors and stockholders. Any such actions could involve one or more of the events referred to in clauses (a) through (j) of Item 4 of Schedule 13D, including, potentially, one or more mergers, consolidations, sales or acquisitions of assets, changes in control, issuances, purchases, dispositions or pledges of securities or other changes in capitalization.

Aggregate number of shares beneficially owned by the reporting persons: EcoR1: 22,970,388 Oleg Nodelman: 22,970,388 Qualified Fund: 21,582,563 Aggregate percentage of the class of Common Stock beneficially owned by the reporting persons: EcoR1: 30.7% Oleg Nodelman: 30.7% Qualified Fund: 28.8%

Number of shares as to which the each reporting person has: (i) Sole power to vote or to direct the vote: EcoR1: 0 Oleg Nodelman: 0 Qualified Fund: 0 (ii) Shared power to vote or to direct the vote: EcoR1: 22,970,388 Oleg Nodelman: 22,970,388 Qualified Fund: 21,582,563 (iii) Sole power to dispose or to direct the disposition of: EcoR1: 0 Oleg Nodelman: 0 Qualified Fund: 0 (iv) Shared power to dispose or to direct the disposition of: EcoR1: 22,970,388 Oleg Nodelman: 22,970,388 Qualified Fund: 21,582,563

The reporting persons have not engaged in any transactions in the Issuer's Common Stock in the 60 days preceding the date of this Schedule 13D.

Not applicable.

Not applicable.

EcoR1 is the investment adviser to its clients pursuant to investment management agreements or limited partnership agreements providing to EcoR1 the authority, among other things, to invest the funds of such clients in Stock, to vote and dispose of Stock and to file this statement on behalf of such clients. Pursuant to such limited partnership agreements, EcoR1 is entitled to allocations based on assets under management and realized and unrealized gains. Pursuant to such investment management agreements, EcoR1 is entitled to fees based on assets under management and realized and unrealized gains.

Exhibit 99.1 - Agreement Regarding Joint Filing of Statement on Schedule 13D or 13G.