ABOUT THIS PROSPECTUS SUPPLEMENT
This prospectus supplement and the accompanying prospectus are part of a registration statement that we have filed with the Securities and
Exchange Commission (the SEC), utilizing a shelf registration process. Under the shelf registration process, we may, from time to time, offer and sell any combination of our securities described in the accompanying prospectus
in one or more offerings. We are providing information to you about this offering of our common stock in two parts. The first part is this prospectus supplement, which provides you with specific information regarding the terms of this offering and
our common stock, and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus. The second part is the accompanying
prospectus, which provides more general information, some of which does not apply to this offering of our common stock.
We provide
information to you about this offering of shares of our common stock in this prospectus supplement, which describes the specific details regarding this offering. This prospectus supplement adds to, updates, and changes information contained in the
accompanying prospectus and the information incorporated by reference therein with respect to this offering of our common stock. To the extent that any information in this prospectus supplement is inconsistent with the information in the
accompanying prospectus or any information incorporated by reference therein, you should rely on the information in this prospectus supplement. If any statement in one of these documents is inconsistent with a statement in another document having a
later datefor example, a document incorporated by reference in this prospectus supplementthe statement in the document having the later date modifies or supersedes the earlier statement as our business, financial condition, results of
operations, and prospects may have changed since the earlier dates.
Before buying any of the common stock that we are offering, we urge
you to carefully read this prospectus supplement and the accompanying prospectus and all of the information incorporated by reference herein and therein, as well as the additional information described under the sections titled Where You Can
Find More Information and Incorporation of Certain Information by Reference. These documents contain important information that you should consider when making your investment decision.
We have not, and the underwriter has not, authorized any other person to provide you with any information different from that contained or
incorporated by reference in this prospectus supplement and in the accompanying prospectus or in any free writing prospectus prepared by us or on our behalf. We and the underwriter take no responsibility for, and can provide no assurance as to the
reliability of, any other information that others may give you. We are not, and the underwriter is not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information
appearing in this prospectus supplement or the accompanying prospectus, in the documents incorporated by reference herein or therein or in any free writing prospectus is accurate only as of the date of such document. Our business, financial
condition, results of operations, and prospects may have changed since that date.
We are offering to sell, and seeking offers to buy,
shares of common stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United
States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement
and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities
offered by this prospectus supplement or the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.
Unless the context indicates otherwise, as used in this prospectus supplement and the accompanying prospectus, the terms Alector,
Alector, Inc., Company, we, us and our refer to Alector, Inc. and, where appropriate, its subsidiaries on a consolidated basis.
This prospectus supplement and the accompanying prospectus and the information incorporated herein and therein by reference include
trademarks, service marks, and trade names owned by us or other companies. All trademarks, service marks, and trade names included or incorporated by reference into this prospectus supplement and the accompanying prospectus are the property of their
respective owners.
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