Footnotes1 Based on 2020 public filings from Regeneron, Roche and NovartisForward-looking StatementsStatements contained in this letter regarding the events or results that may occur in the future are “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. Such statements include, but are not limited to, statements regarding Adverum’s expectations that its two global Phase 3 trials in wet AMD will initiate in parallel in the fourth quarter of 2021, that it will report clinical data and achieve pre-commercialization, clinical and development milestones throughout the rest of 2021 and beyond, and that it will name a high-quality independent director with commercial gene therapy experience in 2021 and submit a BLA to the FDA in 2024. Actual results could differ materially from those anticipated in such forward-looking statements as a result of various risks and uncertainties, which include risks inherent to, without limitation: Adverum’s novel technology, which makes it difficult to predict the time and cost of product candidate development and obtaining regulatory approval; the results of early clinical trials not always being predictive of future results; and the potential for future complications or side effects in connection with use of ADVM-022. Risks and uncertainties facing Adverum are described more fully in Adverum’s Annual Report on Form 10-K for the year ended December 31, 2020 and any subsequent filings with the SEC under the heading “Risk Factors.” All forward-looking statements contained in this letter speak only as of the date on which they were made. Adverum undertakes no obligation to update such statements to reflect events that occur or circumstances that exist after the date on which they were made.Important InformationAdverum Biotechnologies, Inc. (“Adverum”) has filed a definitive proxy statement and form of associated WHITE proxy card with the U.S. Securities and Exchange Commission (the “SEC”) in connection with the solicitation of proxies for Adverum’s 2021 Annual Meeting (the “Proxy Statement”). Adverum, its directors and certain of its executive officers and employees will be participants in the solicitation of proxies from stockholders in respect of the 2021 Annual Meeting. Information regarding the names of Adverum’s directors, executive officers and employees and their respective interests in Adverum by security holdings or otherwise is set forth in the Proxy Statement. Details concerning the nominees of Adverum’s Board of Directors for election at the 2021 Annual Meeting are included in the Proxy Statement.BEFORE MAKING ANY VOTING DECISION, INVESTORS AND STOCKHOLDERS OF ADVERUM ARE URGED TO READ ALL RELEVANT DOCUMENTS FILED WITH OR FURNISHED TO THE SEC, INCLUDING THE ADVERUM’S DEFINITIVE PROXY STATEMENT AND ANY SUPPLEMENTS THERETO AND ACCOMPANYING WHITE PROXY CARD, BECAUSE THEY CONTAIN IMPORTANT INFORMATION. Investors andstockholders can obtain a copy of the Proxy Statement and other relevant documents filed by Adverum free of charge from the SEC’s website, www.sec.gov. Stockholders may also contact Innisfree M&A Incorporated with questions or requests for additional copies of the proxy materials by calling toll free at (877) 750-9496. Your Vote Is Important, No Matter How Many or How Few Shares You Own If you have any questions, or need assistance in voting your shares on the WHITE proxy card, please call the firm assisting Adverum with the solicitation of proxies: INNISFREE M&A INCORPORATEDStockholders Call Toll-Free at (877) 750-9496Banks and Brokers Call Collect (212) 750-5833