Portolan Capital amends 13G for Castle Biosciences stake

Portolan Capital Management, LLC and manager George McCabe amended a Schedule 13G reporting beneficial ownership of 1,350,283 shares of Castle Biosciences common stock as of 03/31/2026.

Rhea-AI Filing Summary

Portolan Capital Management, LLC and manager George McCabe amended a Schedule 13G reporting beneficial ownership of 1,350,283 shares of Castle Biosciences common stock as of 03/31/2026. The filing shows 4.45% ownership and indicates sole voting and dispositive power over those shares. The amendment is signed by Mr. McCabe on 05/15/2026.

Insights

Institutional Ownership Analyst

Amendment confirms a sub-5% passive stake by Portolan and managerial attribution.

Portolan Capital Management, LLC and George McCabe report beneficial ownership of 1,350,283 shares (4.45%) as of 03/31/2026. The cover rows show sole voting and dispositive power, indicating control over voting and sale decisions for this block.

Timing: the amendment is signed 05/15/2026. Cash-flow treatment or planned transactions are not stated; subsequent filings would disclose changes in position.

Regulatory Compliance Specialist

Form classification and percent threshold are consistent with Schedule 13G reporting.

The filing is titled an amendment (13G/A) and states Ownership of 5 Percent or Less of a Class, aligning with passive/Section 13 reporting thresholds. The cover-page rows referenced supply the numeric details for Item 4 ownership, voting, and dispositive power.

Signatures dated 05/15/2026 finalize the amendment. No exemptions, plans, or changes to beneficial ownership method are disclosed here.

Key Figures

Shares beneficially owned: 1,350,283 shares Percent of class: 4.45% Signature date: 05/15/2026

3 metrics

Shares beneficially owned 1,350,283 shares Amount reported in cover-row 9 as of 03/31/2026

Percent of class 4.45% Percent reported in cover-row 11 as of 03/31/2026

Signature date 05/15/2026 Amendment signed by George McCabe

Key Terms

Schedule 13G/A, Beneficially owned, Sole dispositive power

3 terms

Schedule 13G/A regulatory

"Amendment No. 1; Item 1. Name of issuer: Castle Biosciences, Inc."

A Schedule 13G/A is an amended public filing with the U.S. securities regulator that updates a previous Schedule 13G, disclosing when an individual or group holds a substantial (typically over 5%) stake in a company and is claiming a passive, non‑controlling intent. Investors monitor these updates because rising or falling holdings can signal changing confidence, potential future moves, or shifts in voting power — like watching a public ledger where large shareholders quietly adjust their positions.

Beneficially owned regulatory

"This statement is being filed with respect to the shares Common Stock of the Issuer beneficially owned"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

Sole dispositive power regulatory

"Sole Dispositive Power 1,350,283.00"

Sole dispositive power is the exclusive legal authority to decide what happens to a security — for example, whether to sell, transfer, or retain shares — without needing anyone else’s permission. Investors care because it signals who truly controls the economic outcome of an investment: like holding the only key to a safe, the holder can realize gains or losses and may trigger regulatory reporting, insider rules, or influence over corporate ownership.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What stake does Portolan Capital report in Castle Biosciences (CSTL)?

Portolan Capital reports beneficial ownership of 1,350,283 shares, representing 4.45% of Castle Biosciences as of 03/31/2026. The filing attributes sole voting and dispositive power to Portolan and indirectly to George McCabe as manager.

Who is the reporting person on the 13G/A for CSTL?

The reporting persons are Portolan Capital Management, LLC (as investment manager) and George McCabe (Manager). The address listed is 2 International Place, FL 26, Boston, MA 02110, and citizenships are Delaware and United States.

What date is the ownership measured and when was the amendment signed?

The ownership figure is reported as of 03/31/2026. The amendment is signed by George McCabe on 05/15/2026, reflecting an updated disclosure filed after the measurement date.

Does the filing indicate Portolan is a passive holder or an active investor?

The form is a Schedule 13G amendment stating "Ownership of 5 Percent or Less of a Class," which is consistent with passive reporting. The filing shows sole voting and dispositive power but does not state active control or intent to influence management.

Are there shared voting or dispositive powers reported in the 13G/A?

No. The cover rows and Item 4(c) indicate Shared Voting Power: 0 and Shared Dispositive Power: 0, with sole voting and dispositive power equal to 1,350,283 shares.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Accepted by SEC EDGAR 05/15/2026 - 04:10 PM EDT Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 1)

Castle Biosciences, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

14843C105

(CUSIP Number)

03/31/2026

(Date of Event Which Requires Filing of this Statement)

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

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 14843C105

1

Names of Reporting Persons

Portolan Capital Management, LLC

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 1,350,283.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 1,350,283.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,350,283.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

4.45 %

12

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13G

CUSIP Number(s): 14843C105

1

Names of Reporting Persons

George McCabe

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 1,350,283.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 1,350,283.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,350,283.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

4.45 %

12

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Castle Biosciences, Inc.
(b)	Address of issuer's principal executive offices: 1500 W. Parkwood Ave, Suite 400, Friendswood, TX 77546
Item 2.
(a)	Name of person filing: This statement is being filed with respect to the shares Common Stock of the Issuer beneficially owned (1) directly by Portolan Capital Management, LLC, a registered investment adviser, in its capacity as investment manager for various clients, and (2) indirectly by George McCabe, the Manager of Portolan Capital Management, LLC. Portolan Capital Management, LLC and Mr. McCabe are sometimes individually referred to herein as a "Reporting Person" and collectively as the "Reporting Persons."
(b)	Address or principal business office or, if none, residence: The address of the principal business office of each of the Reporting Persons is 2 International Place, FL 26, Boston, MA 02110
(c)	Citizenship: Portolan Capital Management, LLC - DE Mr. McCabe - USA
(d)	Title of class of securities: Common Stock, $0.001 par value per share
(e)	CUSIP No.: 14843C105
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: The information required by Item 4(a) is set forth in Row 9 of the cover page for each of the Reporting Persons and is incorporated by reference herein.
(b)	Percent of class: The information required by Item 4(b) is set forth in Row 11 of the cover page for each of the Reporting Persons and is incorporated by reference herein.
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: The information required by Item 4(c) is set forth in Row 5 of the cover page for each of the Reporting Persons and is incorporated by reference herein.
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: The information required by Item 4(c) is set forth in Row 7 of the cover page for each of the Reporting Persons and is incorporated by reference herein.
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	Ownership of 5 Percent or Less of a Class.
	Ownership of 5 percent or less of a class
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	Not Applicable
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11.

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.

Portolan Capital Management, LLC

Signature:	George McCabe
Name/Title:	George McCabe/Manager
Date:	05/15/2026

George McCabe

Signature:	George McCabe
Name/Title:	George McCabe
Date:	05/15/2026