YORK CHARLES N II (COO, CFO and Secretary) sold 1.4% of their position (4.06K shares) of DAY ONE BIOPHARMACEUTICALS, INC. on the open market

Open‑market sale worth $36.2K — a minor trim. Review remaining holdings and any 10b5‑1 plan disclosures.

S · 1 sale

YORK CHARLES N II

Filed: 11/18/2025ACC: 0001655962-25-000006

InsiderYORK CHARLES N IICIK 1655962

→

IssuerDAY ONE BIOPHARMACEUTICALS, INC.CIK 1845337

Officer· COO, CFO and Secretary

Since May 2021

-4.06K

Net shares

Net seller

$36.2K

Total sold

Open market trades

0B · 1S

Total transactions

16.88K acquired · 20.94K disposed

Non-Derivative Transactions5

Common Stock

Direct

M · Option Exercise

+1.38K

Price / share

—

Trade value

—

% of position

0.5%

11/15/2025

281.90K→283.28K

Common Stock

Direct

M · Option Exercise

+2.25K

Price / share

—

Trade value

—

% of position

0.8%

11/15/2025

283.28K→285.53K

Common Stock

Direct

M · Option Exercise

+6.63K

Price / share

—

Trade value

—

% of position

2.3%

11/15/2025

285.53K→292.15K

Common Stock

Direct

M · Option Exercise

+6.63K

Price / share

—

Trade value

—

% of position

2.3%

11/15/2025

292.15K→298.78K

Common Stock

Direct

S · Sale

−4.06K

Price / share

$8.91

Trade value

$36.21K

% of position

1.4%

11/17/2025

298.78K→294.71K

Total

Net shares

+12.81K

Net value

-$36.21K

Security	Date	Type	Shares	Price / Share	Trade Value	% of Position	Held Before	Held After
Common Stock Direct	11/15/2025	M · Option Exercise	+1.38K	—	—	0.5%	281.90K	283.28K
Common Stock Direct	11/15/2025	M · Option Exercise	+2.25K	—	—	0.8%	283.28K	285.53K
Common Stock Direct	11/15/2025	M · Option Exercise	+6.63K	—	—	2.3%	285.53K	292.15K
Common Stock Direct	11/15/2025	M · Option Exercise	+6.63K	—	—	2.3%	292.15K	298.78K
Common Stock Direct	11/17/2025	S · Sale	−4.06K	$8.91	$36.21K	1.4%	298.78K	294.71K
Total			+12.81K		-$36.21K

Derivative Transactions4

Restricted Stock Unit (RSU)

Direct

M · Option Exercise

Converts to

Common Stock

1.38K shares

−1.38K

Strike price

Held after

0.00

Exercisable

—

11/15/2025

Restricted Stock Unit (RSU)

Direct

M · Option Exercise

Converts to

Common Stock

2.25K shares

−2.25K

Strike price

Held after

Exercisable

—

11/15/2025

Restricted Stock Unit (RSU)

Direct

M · Option Exercise

Converts to

Common Stock

6.63K shares

−6.63K

Strike price

Held after

53K

Exercisable

—

11/15/2025

Restricted Stock Unit (RSU)

Direct

M · Option Exercise

Converts to

Common Stock

6.63K shares

−6.63K

Strike price

Held after

79.50K

Exercisable

—

11/15/2025

Security	Converts To	Date	Type	Shares	Exercisable	Held After
Restricted Stock Unit (RSU) Direct	Common Stock1.38K shares	11/15/2025	M · Option Exercise	−1.38K	—	0.00
Restricted Stock Unit (RSU) Direct	Common Stock2.25K shares	11/15/2025	M · Option Exercise	−2.25K	—	9K
Restricted Stock Unit (RSU) Direct	Common Stock6.63K shares	11/15/2025	M · Option Exercise	−6.63K	—	53K
Restricted Stock Unit (RSU) Direct	Common Stock6.63K shares	11/15/2025	M · Option Exercise	−6.63K	—	79.50K