Alector : Corporate Presentation January 2022

Alector Company Overview

January 2022

Forward-Looking Statements

This presentation contains forward-looking statements that involve substantial risks and uncertainties. All statements other than statements of historical facts contained in this presentation are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as "anticipate," "believe," "continue," "could," "estimate," "expect," "intend," "may," "plan," "potentially," "predict," "should," "will" or the negative of these terms or other similar expressions. Forward-looking statements contained in this presentation also include, but are not limited to, statements regarding: our future financial condition, results of operations, business strategy and plans, plans, timelines and expectations related to our product candidates and our other clinical and pre-clinical programs, including with respect to the availability of data, the initiation of future clinical trials and plans and expectations regarding planned regulatory filings with respect to such programs; and objectives of management for future operations, as well as statements regarding industry trends.

We have based these forward-looking statements largely on our current expectations and projections about future events and trends that we believe may affect our financial condition, results of operations, business strategy and financial

needs. These forward-looking statements are subject to a number of risks, uncertainties and assumptions, including, among other things: Alector's plans relating to the development and manufacturing of its product candidates and research programs; the ability of Alector, Inc.'s ("Alector") clinical trials to demonstrate safety and efficacy of its product candidates, and other positive results; the timing and focus of Alector's future clinical trials, and the reporting of data from those trials; Alector's plans relating to commercializing its product candidates, if approved, including the geographic areas of focus and sales strategy; the expected potential benefits of strategic collaborations with third parties and Alector's ability to attract collaborators with development, regulatory and commercialization expertise; Alector's estimates of the number of patients in the United States who suffer from the diseases it is targeting and the number of patients that will enroll in its clinical trials; the size of the market opportunity for Alector's product candidates in each of the diseases it is targeting; Alector's ability to expand its product candidates into additional indications and patient populations; the success of competing therapies that are or may become available; the beneficial characteristics, safety, efficacy, and therapeutic effects of Alector's product candidates; the timing or likelihood of regulatory filings and approvals, including Alector's expectation to seek special designations, such as orphan drug designation, for its product candidates for various diseases; Alector's ability to obtain and maintain regulatory approval of its product candidates; Alector's plans relating to the further development and manufacturing of its product candidates, including additional indications that it may pursue; existing regulations and regulatory developments in the United States and other jurisdictions; Alector's continued reliance on

third parties to conduct additional clinical trials of its product candidates, and for the manufacture of its product candidates for preclinical studies and clinical trials; and the other risks, uncertainties and assumptions discussed in the public filings we have made and will make with the Securities and Exchange Commission ("SEC"). These risks are not exhaustive. New risk factors emerge from time to time and it is not possible for our management to predict all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in, or implied by, any forward-looking statements. You should not rely upon forward-looking statements as predictions of future events. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements.

This presentation also contains results based on data from our clinical trials. These clinical trials are ongoing and this presentation does not speak to, and you should make no assumptions about, any additional data. In addition, the information we have chosen to publicly disclose regarding our product candidates has been selected from a more extensive amount of available information. You or others may not agree with what we determine is the material or otherwise appropriate information to include in our disclosure, and any information we determine not to disclose may ultimately be deemed significant with respect to future decisions, conclusions, views, activities or otherwise. If the initial data that we report differ from updated, late, final or actual results, or if others, including regulatory authorities, disagree with the conclusions reached, our ability to obtain approval for, and commercialize our other product candidates may be harmed, which could harm our business, financial condition, results of operations and prospects.

Except as required by law, we undertake no obligation to update any statements in this presentation for any reason after the date of this presentation.

We have filed Current Reports on Form 8-K, Quarterly Reports on Form 10-Q, Annual Reports on Form 10-K, and other documents with the SEC. You should read these documents for more complete information about us. You may obtain these documents for free by visiting EDGAR on the SEC website at www.sec.gov.

This presentation discusses certain AL001 and other investigational therapeutic agents, which have not yet been approved for marketing by the U.S. Food and Drug Administration. No representation is made as to the safety or effectiveness of our product candidate for the therapeutic use for which it is being studied.

This presentation contains statistical data based on independent industry publications or other publicly available information, as well as other information based on our internal sources. We have not independently verified the accuracy or completeness of the data contained in these industry publications and other publicly available information. Accordingly, we make no representations as to the accuracy or completeness of that data.

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At Alector, we envision a world where each individual retains his or her full brain function and cognitive faculties throughout life-a world where dementia and neurodegeneration are

illnesses of the past."

Arnon Rosenthal, PhD, Chief Executive Officer, Co-Founder

Rapidly translating scientific insights into a broad portfolio of first-in-class programs

1 novel approach	candidates in	more candidates
4 clinical trials	3 entering the clinic

3 collaborativepartners

• Founded to pioneer a new field of research: Immuno-neurology
• Informed by neuroscience, human genetics and immunology
• Substantial IP portfolio established
• • 20 issued patents
  • 380+ patent applications

• Clinical-stageprograms targeting progranulin, TREM2 and SIGLEC3
• Potential treatments in development for FTD (Phase 3), ALS (Phase 2),
  Alzheimer's (Phase 2) + Parkinson's and solid tumors (upcoming)

• Emerging programs in immuno-neurology and immuno-oncology readying for INDs in 2022
• ~A dozen more innate immunology research programs in early evaluation
• Continued investment in research and discovery

• Agreements in place with GSK, Abbvie and Innovent
• Retained significant rights in the U.S.

MILLIONS OF PATIENTS WAITING

FTD = Frontotemporal dementia
ALS= Amyotrophic lateral sclerosis	4

Pioneering immuno-neurology

Human Genetics	Immunology	Neuroscience
Genetic risk factors for	Our therapeutics	Measuring biomarkers of
neurodegenerative diseases	target microglia cells,	neurodegeneration enhance
point to neuroinflammation	regulators of the brain's	probability of success
as a causal factor	immune system

IMMUNO-NEUROLOGY

Recruiting the brain's immune system

to address neurodegeneration

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