Quantum-Si founder's trusts plan sale of up to 8.95M shares

Quantum-Si Inc (QSI) is the subject of this amended Schedule 13D, which updates the beneficial ownership and trading plans of its founder, Jonathan M. Rothberg, Ph.D.

Rhea-AI Filing Summary

Quantum-Si Inc (QSI) is the subject of this amended Schedule 13D, which updates the beneficial ownership and trading plans of its founder, Jonathan M. Rothberg, Ph.D. Rothberg is reported to beneficially own 14,376,415 shares of Class A common stock, representing 7.20% of the 198,946,412 Class A shares outstanding as of August 11, 2026. He also beneficially owns 19,937,500 shares of Class B common stock, representing 100% of the Class B shares outstanding.

The Class A holdings include shares held directly by Rothberg, options exercisable within 60 days, and shares held by entities owned by trusts for his children, as well as shares held by his spouse and by 23rd Century Capital LLC. In connection with estate planning, entities owned by these trusts have entered into a Rule 10b5-1 trading plan under which sales of up to 8,947,746 Class A shares may occur between January 15, 2027 and January 15, 2028, subject to minimum price parameters, with no assurance that any sales will occur.

Key Figures

Class A shares beneficially owned by Jonathan M. Rothberg: 14,376,415 shares of Class A common stock Class A ownership percentage of Jonathan M. Rothberg: 7.20% Class A shares outstanding: 198,946,412 shares of Class A common stock +5 more

8 metrics

Class A shares beneficially owned by Jonathan M. Rothberg 14,376,415 shares of Class A common stock Beneficial ownership as reported in the Schedule 13D/A

Class A ownership percentage of Jonathan M. Rothberg 7.20% Based on 198,946,412 Class A shares outstanding as of August 11, 2026

Class A shares outstanding 198,946,412 shares of Class A common stock Outstanding as of August 11, 2026

Class B shares beneficially owned by Jonathan M. Rothberg 19,937,500 shares of Class B common stock Beneficial ownership; all Class B shares outstanding as of August 11, 2026

Class B ownership percentage of Jonathan M. Rothberg 100.00% Based on 19,937,500 Class B shares outstanding as of August 11, 2026

Shares in trusts’ Rule 10b5-1 plan 8,947,746 shares of Class A common stock Maximum shares that may be sold under the Rule 10b5-1 plan

Plan sales period start date January 15, 2027 Beginning of the Rule 10b5-1 plan sales period after the cooling-off period

Plan sales period end date January 15, 2028 End of the Rule 10b5-1 plan sales period

Key Terms

beneficially owned, Rule 10b5-1, cooling-off period, Class B common stock, +1 more

5 terms

beneficially owned financial

"Aggregate amount beneficially owned by each reporting person 14,376,415.00"

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.

Rule 10b5-1 regulatory

"entered into a trading plan pursuant to Rule 10b5-1 , as amended"

Rule 10b5-1 is a regulation that allows company insiders to buy or sell their shares at predetermined times, even if they have access to non-public information. It acts like setting a schedule in advance for transactions, helping prevent accusations of unfair trading. This rule provides a way for insiders to plan trades transparently, giving investors confidence that these transactions are not based on hidden information.

cooling-off period regulatory

"beginning following the cooling-off period contained in Rule 10b5-1(c)"

A cooling-off period is a temporary pause around a planned securities offering or major corporate action during which the company, underwriters and insiders limit promotional statements and certain transactions so investors can review official documents and regulators can assess disclosures. It matters to investors because it reduces the risk of decisions driven by hype, gives time to read the prospectus and legal filings, and helps ensure a fairer, more orderly market — like a mandatory waiting period before a big sale so buyers can compare facts calmly.

Class B common stock financial

"Class B common stock, par value $0.0001 per share"

A class B common stock is one of multiple types of a company’s ordinary shares that carries specific rights—often different voting power or dividend priority—compared with other classes. For investors it matters because those differences affect how much influence you have over company decisions, the income you might receive, and how freely the shares trade; think of it like owning a car with different keys: some keys let you start the engine and open the trunk, others only unlock the door.

trading plan financial

"have entered into a trading plan pursuant to Rule 10b5-1"

A trading plan is a written set of rules an investor follows about what to buy or sell, when to enter and exit positions, and how much risk to accept—like a travel itinerary that maps the route, stops, and budget before a trip. It matters because it helps remove emotional decisions during market swings, enforces discipline, and makes performance easier to review and improve, reducing the chance of costly impulsive moves.

FAQ

How many Quantum-Si (QSI) Class A shares does Jonathan Rothberg beneficially own?

Jonathan M. Rothberg beneficially owns 14,376,415 shares of Class A common stock of Quantum-Si, representing 7.20% of the Class A shares outstanding, based on 198,946,412 Class A shares outstanding as of August 11, 2026.

What is Jonathan Rothberg’s ownership of Quantum-Si (QSI) Class B common stock?

Jonathan M. Rothberg beneficially owns 19,937,500 shares of Class B common stock of Quantum-Si, representing 100% of the Class B class, based on 19,937,500 Class B shares outstanding as of August 11, 2026.

What trading plan related to Quantum-Si (QSI) shares is disclosed in this amendment?

Entities owned by trusts for Jonathan Rothberg’s children entered a Rule 10b5-1 trading plan that permits potential sales of up to 8,947,746 Class A shares during a plan sales period from January 15, 2027 to January 15, 2028, subject to minimum price parameters.

Are any Quantum-Si (QSI) shares guaranteed to be sold under the new Rule 10b5-1 plan?

No. The filing states there is no assurance that any shares of Class A common stock will be sold under the Rule 10b5-1 trading plan, as all sales are subject to the plan’s terms and minimum price parameters.

How many Quantum-Si (QSI) Class A shares in the plan are held by trusts for Rothberg’s children?

The filing states that 8,947,746 shares of Quantum-Si Class A common stock are held by entities owned by trusts created for the benefit of Jonathan Rothberg’s children. These shares are the ones that may be sold under the disclosed Rule 10b5-1 trading plan.

What portion of Quantum-Si (QSI) Class A shares does 23rd Century Capital LLC beneficially own?

23rd Century Capital LLC beneficially owns 1,917,067 shares of Quantum-Si Class A common stock, representing 0.96% of the Class A shares, based on 198,946,412 Class A shares outstanding as of August 11, 2026.

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Available on EDGAR 08/28/2026 - 05:36 PM Accepted by SEC EDGAR 08/28/2026 - 05:35 PM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 5)

Quantum-Si Inc

(Name of Issuer)

Class A common stock, par value $0.0001 per share, Class B common stock, par value $0.0001 per share

(Title of Class of Securities)

74765K105

(CUSIP Number)

Jonathan M. Rothberg, Ph.D.
c/o Quantum-Si Incorporated, 29 Business Park Drive
Branford, CT, 06405
(866) 688-7374

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

08/26/2026

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

schemaVersion:

SCHEDULE 13D

CUSIP Number(s): 74765K105

1

Name of reporting person

ROTHBERG JONATHAN M

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 12,185,926.00
8	Shared Voting Power 2,190,489.00
9	Sole Dispositive Power 12,185,926.00
10	Shared Dispositive Power 2,190,489.00

11

Aggregate amount beneficially owned by each reporting person

14,376,415.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

7.20 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

This Reporting Person's Table is for Class A common stock. Rows 7 and 9 consists of (i) 2,568,443 shares of Class A common stock of Quantum-Si Incorporated (the ''Issuer'') held by Jonathan M. Rothberg, Ph.D., (ii) stock options to purchase 669,737 shares of Class A common stock of the Issuer which are exercisable within 60 days of August 26, 2026, held by Dr. Jonathan M. Rothberg and (iii) an aggregate of 8,947,746 shares of Class A common stock of the Issuer held by entities owned by trusts created for the benefit of Dr. Jonathan Rothberg's children. Rows 8 and 10 consists of (i) 1,917,067 shares of Class A common stock of the Issuer held by 23rd Century Capital LLC, and (ii) 273,422 shares of Class A common stock of the Issuer held by Dr. Jonathan Rothberg's spouse, Bonnie E. Gould Rothberg, M.D. Row 13 is calculated based on 198,946,412 shares of Class A common stock of the Issuer outstanding as of August 11, 2026.

SCHEDULE 13D

CUSIP Number(s): 74765K105

1

Name of reporting person

ROTHBERG JONATHAN M.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 19,937,500.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 19,937,500.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

19,937,500.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

100.00 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

This Reporting Person's Table is for Class B common stock. Rows 7 and 9 consists of (i) 17,943,750 shares of Class B common stock of the Issuer held by 4C Holdings I, LLC and (ii) 1,993,750 shares of Class B common stock of the Issuer held by 4C Holdings V, LLC. Row 13 is calculated based on 19,937,500 shares of Class B common stock of the Issuer outstanding as of August 11, 2026.

SCHEDULE 13D

CUSIP Number(s): 74765K105

1

Name of reporting person

23rd Century Capital LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 1,917,067.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,917,067.00

11

Aggregate amount beneficially owned by each reporting person

1,917,067.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0.96 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

This Reporting Person's table is for Class A common stock. Row 13 is calculated based on 198,946,412 shares of Class A common stock of the Issuer outstanding as of August 11, 2026.

SCHEDULE 13D

CUSIP Number(s): 74765K105

1

Name of reporting person

4C Holdings I, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 17,943,750.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 17,943,750.00

11

Aggregate amount beneficially owned by each reporting person

17,943,750.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

90.00 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

This Reporting Person's table is for Class B common stock. Row 13 is calculated based on 19,937,500 shares of Class B common stock of the Issuer outstanding as of August 11, 2026.

SCHEDULE 13D

CUSIP Number(s): 74765K105

1

Name of reporting person

4C Holdings V, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

PF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 1,993,750.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,993,750.00

11

Aggregate amount beneficially owned by each reporting person

1,993,750.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

10.00 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

This Reporting Person's table is for Class B common stock. Row 13 is calculated based on 19,937,500 shares of Class B common stock of the Issuer outstanding as of August 11, 2026.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Class A common stock, par value $0.0001 per share, Class B common stock, par value $0.0001 per share
(b)	Name of Issuer: Quantum-Si Inc
(c)	Address of Issuer's Principal Executive Offices: 29 BUSINESS PARK DRIVE, BRANFORD, CONNECTICUT , 06405.
Item 1 Comment: Explanatory Note Amendment No. 5 to Schedule 13D ("Amendment No. 5") amends and supplements the Schedule 13D dated June 18, 2021, as amended by Amendment No. 1 filed on September 13, 2023, Amendment No. 2 filed on September 17, 2024, Amendment No. 3 filed on December 31, 2024 and Amendment No. 4 filed on September 11, 2025 (collectively, as amended, the "Schedule 13D") relating to the Class A common stock, par value $0.0001 per share ("Class A common stock"), and Class B common stock, par value $0.0001 per share, of Quantum-Si Incorporated, a Delaware corporation. Information reported in the Schedule 13D remains in effect except to the extent that it is amended, restated or superseded by information contained in this Amendment No. 5. Capitalized terms used but not defined herein have the meanings ascribed to them in the Schedule 13D.
Item 4.	Purpose of Transaction
	In connection with estate planning, entities owned by trusts created for the benefit of Dr. Jonathan Rothberg's children have entered into a trading plan pursuant to Rule 10b5-1 under the Securities Exchange Act of 1934, as amended (the "Plan"), a copy of which is attached hereto as Exhibit 6. Pursuant to the Plan, sales of up to 8,947,746 shares of Class A common stock may be effected during the plan sales period beginning following the cooling-off period contained in Rule 10b5-1(c) on January 15, 2027 and ending on January 15, 2028 in accordance with the terms and conditions of the Plan. The sale of shares of Class A common stock under the Plan is subject to minimum price parameters included in the Plan, and there is no assurance that any shares of Class A common stock will be sold under the Plan.
Item 7.	Material to be Filed as Exhibits.
	Item 7 of the Schedule 13D is hereby amended by adding the following exhibit: 6. Rule 10b5-1 Trading Plan

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.

ROTHBERG JONATHAN M

Signature:	/s/ Jonathan M. Rothberg
Name/Title:	Jonathan M. Rothberg, Ph.D.
Date:	08/28/2026

ROTHBERG JONATHAN M.

Signature:	/s/ Jonathan M. Rothberg
Name/Title:	Jonathan M. Rothberg, Ph.D.
Date:	08/28/2026

23rd Century Capital LLC

Signature:	/s/ Jonathan M. Rothberg
Name/Title:	Jonathan M. Rothberg, Ph.D., Member
Date:	08/28/2026

4C Holdings I, LLC

Signature:	/s/ Jonathan M. Rothberg
Name/Title:	Jonathan M. Rothberg, Ph.D., Manager
Date:	08/28/2026

4C Holdings V, LLC

Signature:	/s/ Jonathan M. Rothberg
Name/Title:	Jonathan M. Rothberg, Ph.D., Manager
Date:	08/28/2026