# EDGAR Filing Document

**Accession Number:** 0001493463
**File Stem:** 0001193125-25-280330
**Filing Date:** 2025-11
**Character Count:** 36240
**Document Hash:** 1c5501c254316496a41e77b5a0d20ee4
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001193125-25-280330.hdr.sgml**: 20251113

**ACCESSION NUMBER**: 0001193125-25-280330

**CONFORMED SUBMISSION TYPE**: SCHEDULE 13G/A

**PUBLIC DOCUMENT COUNT**: 1

**FILED AS OF DATE**: 20251113

**DATE AS OF CHANGE**: 20251113

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Alector, Inc.
- **CENTRAL INDEX KEY:** 0001653087
- **STANDARD INDUSTRIAL CLASSIFICATION:** BIOLOGICAL PRODUCTS (NO DIAGNOSTIC SUBSTANCES) [2836]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 822933343
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13G/A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-90858
- **FILM NUMBER:** 251479266

**BUSINESS ADDRESS:**
- **STREET 1:** 131 OYSTER POINT BLVD., SUITE 600
- **CITY:** SOUTH SAN FRANCISCO
- **STATE:** CA
- **ZIP:** 94080
- **BUSINESS PHONE:** 415-231-5660

**MAIL ADDRESS:**
- **STREET 1:** 131 OYSTER POINT BLVD., SUITE 600
- **CITY:** SOUTH SAN FRANCISCO
- **STATE:** CA
- **ZIP:** 94080

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Alector LLC
- **DATE OF NAME CHANGE:** 20150915
**FILED BY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Polaris Venture Partners VI, L.P.
- **CENTRAL INDEX KEY:** 0001493463

**ORGANIZATION NAME:**
- **EIN:** 000000000
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13G/A

**BUSINESS ADDRESS:**
- **STREET 1:** ONE MARINA PARK DRIVE, 8TH FLOOR
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02210
- **BUSINESS PHONE:** 781-290-0770

**MAIL ADDRESS:**
- **STREET 1:** ONE MARINA PARK DRIVE, 8TH FLOOR
- **CITY:** BOSTON
- **STATE:** MA
- **ZIP:** 02210

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## SCHEDULE 13G

### Under the Securities Exchange Act of 1934

**(Amendment No. 6)**

**Issuer:** Alector, Inc.

**Title of Class of Securities:** Common Stock, $0.0001 par value per share

**CUSIP Number:** 014442107

**Date of Event Which Requires Filing of this Statement:** 09/30/2025

**Check the appropriate box to designate the rule pursuant to which this Schedule is filed:**

- [ ] Rule 13d-1(b)

- [ ] Rule 13d-1(c)

- [x] Rule 13d-1(d)

---

| 1. | Names of Reporting Persons<br>Polaris Venture Partners VI, L.P. | Names of Reporting Persons<br>Polaris Venture Partners VI, L.P. |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>498468 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>498468 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>0 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>498468 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>498468 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>0.5% | Percent of Class Represented by Amount in Row (9)<br>0.5% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** All shares are held of record by PVP VI (as defined in Item 2(a) of the Original Schedule 13G (as defined below)). PVM VI (as defined in Item 2(a) of the Original Schedule 13G), the general partner of PVP VI, may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of the Issuer's common stock ("Common Stock") outstanding as of October 31, 2025, as set forth in the Issuer's quarterly report on Form 10-Q for the quarter ended September 30, 2025, filed with the United States Securities and Exchange Commission (the "Commission") on November 6, 2025 (the "Form 10-Q").

---

| 1. | Names of Reporting Persons<br>Polaris Venture Partners Founders' Fund VI, L.P. | Names of Reporting Persons<br>Polaris Venture Partners Founders' Fund VI, L.P. |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>378403 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>378403 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>0 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>378403 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>378403 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>0.3% | Percent of Class Represented by Amount in Row (9)<br>0.3% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** All shares are held of record by PVPFF VI (as defined in Item 2(a) of the Original Schedule 13G). PVM VI, the general partner of PVPFF VI, may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Polaris Venture Partners VI (AIV), L.P. | Names of Reporting Persons<br>Polaris Venture Partners VI (AIV), L.P. |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>4797563 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>4797563 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>0 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4797563 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4797563 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>4.4% | Percent of Class Represented by Amount in Row (9)<br>4.4% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** All shares are held of record by PVP VI (AIV) (as defined in Item 2(a) of the Original Schedule 13G). PVM VI, the general partner of PVP VI (AIV), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Polaris Venture Partners VI (AIV-B), L.P. | Names of Reporting Persons<br>Polaris Venture Partners VI (AIV-B), L.P. |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>1179383 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>1179383 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>0 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>1179383 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>1179383 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>1.1% | Percent of Class Represented by Amount in Row (9)<br>1.1% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** All shares are held of record by PVP VI (AIV-B) (as defined in Item 2(a) of the Original Schedule 13G). PVM VI, the general partner of PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

Based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Polaris Venture Management Co VI, L.L.C. | Names of Reporting Persons<br>Polaris Venture Management Co VI, L.L.C. |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6853817 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6853817 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.3% | Percent of Class Represented by Amount in Row (9)<br>6.3% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI, (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>David Barrett | Names of Reporting Persons<br>David Barrett |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>26254 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>26254 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6880071 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6880071 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.3% | Percent of Class Represented by Amount in Row (9)<br>6.3% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 are held directly by David Barrett.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI, (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Brian Chee | Names of Reporting Persons<br>Brian Chee |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>26267 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>26267 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6880084 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6880084 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.3% | Percent of Class Represented by Amount in Row (9)<br>6.3% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 are held directly by Brian Chee.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Jonathan A. Flint | Names of Reporting Persons<br>Jonathan A. Flint |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>39125 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>39125 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6892942 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6892942 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.3% | Percent of Class Represented by Amount in Row (9)<br>6.3% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 are held directly by Jonathan A. Flint.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Terrance G. McGuire | Names of Reporting Persons<br>Terrance G. McGuire |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>241434 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>241434 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>7095251 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>7095251 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.5% | Percent of Class Represented by Amount in Row (9)<br>6.5% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 consist of (i) 70,719 shares held directly by Terrance G. McGuire, (ii) 144,465 shares held by McGuire Family Trust, and (iii) 26,250 shares consist of restricted stock units that have been granted to Terrance G. McGuire in his capacity as a former director of the Issuer. Terrance G. McGuire is the donor of the McGuire Family Trust and may be deemed to share voting, investment and dispositive power with respect to the shares held by the McGuire Family Trust.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,177,722 shares of Common Stock, calculated as follows: (i) 26,250 shares that represent restricted stock units that have been granted to Mr. McGuire in his capacity as a former director of the Issuer plus (ii) 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Amir Nashat | Names of Reporting Persons<br>Amir Nashat |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>78800 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>78800 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6932617 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6932617 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.4% | Percent of Class Represented by Amount in Row (9)<br>6.4% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 are held directly by Amir Nashat.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI, (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

| 1. | Names of Reporting Persons<br>Bryce Youngren | Names of Reporting Persons<br>Bryce Youngren |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>91332 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>6853817 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>91332 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>6853817 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6945149 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>6945149 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.4% | Percent of Class Represented by Amount in Row (9)<br>6.4% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** The shares reported in Rows 5 and 7 are held directly by Bryce Youngren.

The shares reported in Rows 6 and 8 consist of (i) 498,468 shares held of record by PVP VI, (ii) 378,403 shares held of record by PVPFF VI (iii) 4,797,563 shares held of record by PVP VI (AIV) and (iv) 1,179,383 shares held of record by PVP VI (AIV-B). PVM VI, the general partner of each of PVP VI, PVPFF VI, PVP VI (AIV) and PVP VI (AIV-B), may be deemed to have voting, investment and dispositive power with respect to these securities. David Barrett, Brian Chee, Jonathan A. Flint, Terrance G. McGuire, Amir Nashat, and Bryce Youngren are the managing members of PVM VI and may each be deemed to share voting, investment and dispositive power with respect to these securities.

The percentage in Row 11 is based on 109,151,472 shares of Common Stock outstanding as of October 31, 2025, as set forth in the Form 10-Q.

---

**Item 1(a). Name of Issuer:**
Alector, Inc.

**Item 1(b). Address of Issuer's Principal Executive Offices:**
131 OYSTER POINT BLVD., SUITE 600, SOUTH SAN FRANCISCO, CA, 94080

**Item 2(a). Name of Person Filing:**
This Amendment No. 6 (this "Amendment") amends and supplements the Schedule 13G initially filed with the Commission on February 12, 2020, as amended by Amendment No. 1 filed with the Commission on February 12, 2021, Amendment No. 2 filed with the Commission on February 11, 2022, Amendment No. 3 filed with the Commission on February 7, 2023, Amendment No. 4 filed with the Commission on February 9, 2024 and Amendment No. 5 filed with the Commission on November 12, 2024 (collectively, the "Original Schedule 13G"). Only those items that are hereby reported are amended; all other items reported in the Original Schedule 13G remain unchanged. Information given in response to each item shall be deemed incorporated by reference in all other items, as applicable. Capitalized terms not defined in this Amendment have the meanings ascribed to them in the Original Schedule 13G.

The following information in Item 4 with respect to the ownership of the Issuer's Common Stock by the Reporting Persons filing this Amendment No. 6 is provided as of November 6, 2025. Each of the Reporting Persons disclaims beneficial ownership of the shares of Common Stock reported in this Amendment No. 6, except for the shares, if any, such Reporting Person holds of record and to the extent of such Reporting Person's pecuniary interest therein.

**Item 2(b). Address of Principal Business Office:**
—

**Item 2(c). Citizenship:**
—

**Item 2(d). Title of Class of Securities:**
Common Stock, $0.0001 par value per share

**Item 2(e). CUSIP Number:**
014442107

**Item 3. If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:**

(a) [ ] Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b) [ ] Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c) [ ] Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d) [ ] Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).

(e) [ ] An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);

(f) [ ] An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);

(g) [ ] A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h) [ ] A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i) [ ] A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j) [ ] A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), if filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

(k) [ ] Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).

**Item 4. Ownership:**

**(a) Amount beneficially owned:**

See Row 9 of cover page for each Reporting Person and the corresponding footnotes.

**(b) Percent of class:**

See Row 11 of cover page for each Reporting Person and the corresponding footnotes.

**(c) Number of shares as to which the person has:**

**(i) Sole power to vote or to direct the vote:**

See Row 5 of cover page for each Reporting Person and the corresponding footnotes.

**(ii) Shared power to vote or to direct the vote:**

See Row 6 of cover page for each Reporting Person and the corresponding footnotes.

**(iii) Sole power to dispose or to direct the disposition of:**

See Row 7 of cover page for each Reporting Person and the corresponding footnotes.

**(iv) Shared power to dispose or to direct the disposition of:**

See Row 8 of cover page for each Reporting Person and the corresponding footnotes.

**Item 5. Ownership of Five Percent or Less of a Class.**

[ ] If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following.

**Item 6. Ownership of More than 5 Percent on Behalf of Another Person.**

—

**Item 7. Identification and Classification of the Subsidiary**

—

**Item 8. Identification and Classification of Members of the Group**

—

**Item 9. Notice of Dissolution of Group**

—

**Item 10. Certification:**

—

### SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Polaris Venture Partners VI, L.P.**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Authorized Signatory of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI, L.P.

**Polaris Venture Partners Founders' Fund VI, L.P.**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Authorized Signatory of Polaris Venture Management Co. VI, L.L.C., GP of Polaris Venture Partners Founders' Fund VI, L.P.

**Polaris Venture Partners VI (AIV), L.P.**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Authorized Signatory of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI (AIV), L.P.

**Polaris Venture Partners VI (AIV-B), L.P.**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Authorized Signatory of Polaris Venture Management Co. VI, L.L.C., general partner of Polaris Venture Partners VI (AIV-B), L.P.

**Polaris Venture Management Co VI, L.L.C.**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Authorized Signatory*

**David Barrett**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact

**Brian Chee**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact

**Jonathan A. Flint**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact

**Terrance G. McGuire**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact

**Amir Nashat**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact

**Bryce Youngren**

**Date:** 11/13/2025

**By:** /s/ Lauren Crockett

**Name & Title:** Lauren Crockett/ Attorney-In-Fact