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        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001655962</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0001845337</issuerCik>
            <issuerName>DAY ONE BIOPHARMACEUTICALS INC</issuerName>
            <secFileNumber>001-40431</secFileNumber>
            <issuerAddress>
                <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                <ns2:city>Brisbane </ns2:city>
                <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                <ns2:zipCode>94005</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>6504840899</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>CHARLES YORK</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>8th Floor</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>80000</noOfUnitsSold>
            <aggregateMarketValue>1368000.00</aggregateMarketValue>
            <noOfUnitsOutstanding>87384856</noOfUnitsOutstanding>
            <approxSaleDate>05/01/2024</approxSaleDate>
            <securitiesExchangeName>NASDAQ</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>02/16/2022</acquiredDate>
            <natureOfAcquisitionTransaction>RSA</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>80000</amountOfSecuritiesAcquired>
            <paymentDate>02/16/2022</paymentDate>
            <natureOfPayment>N/A</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>CHARLES N YORK II</name>
                <address>
                    <ns2:street1>2000 Sierra Point Parkway Suite 501 </ns2:street1>
                    <ns2:city>Brisbane </ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94005</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>02/16/2024</saleDate>
            <amountOfSecuritiesSold>2666</amountOfSecuritiesSold>
            <grossProceeds>40651.43</grossProceeds>
        </securitiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>05/01/2024</noticeDate>
            <planAdoptionDates>
                <planAdoptionDate>10/06/2023</planAdoptionDate>
            </planAdoptionDates>
            <signature>/s/ Charles York</signature>
        </noticeSignature>
    </formData>
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144
0001655962
XXXXXXXX
LIVE
0001845337
DAY ONE BIOPHARMACEUTICALS INC
001-40431
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
6504840899
CHARLES YORK
Officer
Common
Morgan Stanley Smith Barney LLC Executive Financial Services
1 New York Plaza
8th Floor
New York
NY
10004
80000
1368000.00
87384856
05/01/2024
NASDAQ
Common
02/16/2022
RSA
Issuer
N
80000
02/16/2022
N/A
N
CHARLES N YORK II
2000 Sierra Point Parkway Suite 501
Brisbane
CA
94005
Common
02/16/2024
2666
40651.43
05/01/2024
10/06/2023
/s/ Charles York