Day One Biopharmaceuticals Day One Biopharmaceuticals Targeted Therapies for People of All Ages May 2023

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Cancer Drug Development for People of All Ages Growing Portfolio and Runway Beyond Clinical Milestones Tovorafenib (DAY101) Lead Program Mission That Creates Value Day One’s mission is to help children with cancer, from day one and every day after Develop medicines for genomically-defined cancers Establish first-in-class position through rapid pediatric registration Expand to adolescent and adult populations in parallel and pursue those opportunities with the same commitment we do for children Investigational, oral, CNS-penetrant pan-RAF inhibitor Being studied as tablets and pediatric-friendly liquid suspension Breakthrough Therapy Designation Rare Pediatric Disease Designation Orphan Drug Designation (US/EU) Two clinical-stage MEKi assets, in-licensed for combination trials Projected cash runway into 2025 Upcoming key FIREFLY-1 milestones Pre-NDA meeting held on April 19, 2023 Remain in position to initiate the NDA submission in Q2 2023 Presentation of new clinical data in oral presentation at ASCO conference Day One Biopharmaceuticals 1NDA data set will include analysis of primary (ORR by RANO-HGG) and secondary (ORR by RAPNO, PFS) efficacy endpoints, safety, and exploratory analyses (including ORR by RANO-LGG).

Product Candidate Indication Preclinical Phase 1 Phase 2 Phase 3 Recent & Anticipated Milestones Tovorafenib (DAY101) Type II Pan-RAF Inhibitor Relapsed pLGG Frontline pLGG RAF-altered solid tumors2 (monotherapy) Pimasertib MEK 1/2 Inhibitor MAPK-altered solid tumors3 (Combo w/tovorafenib) Our Pipeline *Includes patients ≥12 years of age. 1 FIREFLY-1 Arm 1 expected to support registration. 2 DAY101 adult monotherapy Phase 1 dose escalation and expansion trial previously completed. 3 Pimasertib Phase 1 dose escalation and expansion trial previously completed. pLGG, pediatric low-grade glioma. Tovorafenib and Pimasertib are investigational products. Safety and efficacy have not been established by any health authority. Day One Biopharmaceuticals FDA Breakthrough Therapy Designation for relapsed pLGG FDA Orphan Drug Designation for malignant glioma EC Orphan Designation for glioma FDA Rare Pediatric Disease Designation (PRV Eligible) for pLGG FIREFLY-11 (pivotal) FIREFLY-2 (pivotal) FIRELIGHT-1* FIRELIGHT-1* First patient dosed: March 2023 First patient dosed: November 2021 Poster presented: April 2023 First patient dosed: May 2022 Pre-NDA meeting held: April 19, 2023 Remain in position to initiate NDA submission: Q2 2023 New clinical data presentation planned: June 2023

Day One Biopharmaceuticals Tovorafenib (DAY101) Type II Pan-RAF Inhibitor

Pediatric Low-Grade Gliomas (pLGG) Day One Biopharmaceuticals Despite being the most common brain tumor in children, there are no approved agents and no standard-of-care for the majority of patients with relapsed/progressive disease1,2 ~70% of patients will require systemic therapy Patients have a high rate of recurrence and are frequently treated with multiple lines of systemic therapy over the course of their disease The majority of pLGGs are driven by BRAF alterations3 ~85% of BRAF-altered tumors harbor a KIAA1549-BRAF gene fusion ~15% are driven by BRAF V600E mutation Despite low-grade histology and high long-term survival, pLGGs are chronic and relentless1-4 Goal of therapy is to stabilize or shrink tumors while minimizing treatment-associated toxicities from surgery, chemotherapy, and radiation Many patients today suffer profound tumor and treatment-associated morbidity and significant late effects that persist throughout life 6 y/o with large relapsed BRAF fusion-positive optic pathway glioma 1. Ostrum QT et al., Neuro Oncol. 2015; 16(Suppl 10):x1-x36; 2. De Blank P. et al., Curr Opin Pediatr. 2019 Feb; 31(1):21-27. 3. Jones DTW et al., Cancer Res. 2008; 68:8673–77. 4. Traunwieser T et al., Neurooncol Adv. 2020; 2:vdaa094;

Tovorafenib (DAY101) Inhibits Both BRAF Fusions and BRAF V600 Mutations Day One Biopharmaceuticals Tovorafenib (DAY101) is an investigational, oral, selective, CNS-penetrant, type II pan-RAF inhibitor that was designed to inhibit both monomeric and dimeric RAF kinase Activity in tumors driven by both RAF fusions and BRAF V600E mutations Tablet and pediatric-friendly liquid suspension Once weekly dosing Currently approved type I BRAFi are indicated for use in patients with tumors bearing BRAF V600E mutations Type I BRAF inhibitors cause paradoxical MAPK activation in the setting of wild-type RAF, increasing the risk of tumor growth in BRAF fusion-driven RAS-independent activation of the MAPK pathway MAPK pathway RAS RAF MEK ERK Proliferation and survival RAF mutation RAF fusion Proliferation and survival Proliferation and survival Tovorafenib 1. Sun Y et al., Neuro Oncol. 2017; 19: 774–85; 2. Sievart AJ et al., PNAS. 2013; 110:5957-62; 3. Karajannis MA et al., Neuro Oncol 2014;16(10):1408-16

The Current pLGG Treatment Paradigm Reflects the Unrelenting Nature of this Chronic Brain Tumor Day One Biopharmaceuticals Source: Physician Interviews, Bandopadhayay et al. Pediatric Blood Cancer. 2014; Sievert and Fischer. J Child Neurol. 2009; ClearView Analysis. GTR: Gross Total Resection 1Molecular testing of biopsied samples occurs in all patients. Kandels et. al. Retrospective analysis of comprehensive SIOP registry; Hargrave et. al. Phase I/II;Fangusaro et. al. Phase II Because many pLGGs undergo senescence when patients reach their 20s, the goal of therapy is to maximize tumor control while minimizing treatment-associated toxicities from surgery, chemotherapy, and radiation. As a result, a large number of pLGG patients will undergo multiple lines of systemic therapy over the course of their disease. Surgical Intervention Suspected pLGG GTR No Recurrence Eventual Recurrence 80% 20% ~35% Presentation 1L ~35% 2L Response, no recurrence 3L Chemo(90%) Chemo(50%) Chemo(35%) Other(<5%) Other(<5%) Other(<20%) Targeted Tx(5-10%) Targeted Tx(40-50%) Targeted Tx(40-50%) Response, no recurrence ~50% Biopsy Only ~40% ~20% Molecular Testing Biopsy1(>95%) No Biopsy(5%) ≤ Partial Resection ~65% ~50% ~50% Additional lines of therapy

Pivotal Phase 2 Trial Of Monotherapy Tovorafenib (DAY101) in Relapsed or Progressive pLGG (FIREFLY-1) Day One Biopharmaceuticals Primary endpoint: ORR based on RANO-HGG criteria, assessed by blinded independent central review Secondary endpoints: ORR by RAPNO criteria; PFS; safety Exploratory analyses: ORR by RANO-LGG Three arm, open-label, global registrational phase 2 trial Pivotal Arm 1 (recurrent/progressive pLGG): n=69 RANO-evaluable patients aged 6 months to 25 years harboring a KIAA1549-BRAF fusion or BRAF V600 mutation Arm 2 (expanded access recurrent/progressive LGG): patients aged 6 months to 25 years harboring an activating RAF alteration Arm 3 (extracranial solid tumors): patients aged 6 months to 25 years harboring an activating RAF fusion Endpoints (Pivotal Arm 1) Trial Design Day –28 to 0 Study Drug Administration 420mg/m2 QW After Cycle 27: patients may either continue treatment or enter drug holiday period at any time (at discretion of Investigator) Screening C27D1 Enrollment/ Baseline (C1D1) End of Trial Clinical and radiological evaluations at baseline, and every 3rd cycle for pLGG and every 2nd cycle for solid tumors Eligibility evaluation Treatment period: minimum of 2 years or until progression or toxicity/intolerability Abbreviations: C, cycle; D, day; LGG, low-grade glioma; ORR, objective response rate; PFS, progression-free survival. RANO-HGG, response assessment for neuro-oncology-high grade glioma NCT04775485

FIREFLY-1 Baseline Patient Characteristics Day One Biopharmaceuticals Characteristic Topline Data Arm 1 (N=77) Median age, years (range) 8 (2-21) BRAF alteration, n (%) BRAF V600E BRAF Fusion† 13 (17) 64 (83) Median number of lines of prior therapy (range) 3 (1-9) Prior MAPK pathway targeted therapy, n (%) Yes No 46 (60) 31 (40) Geography, n (%) U.S. Ex-U.S. 27 (35) 50 (65) BRAF alteration (n=77) Location (n=77) Optic pathway 51% BRAF V600E 17% BRAF fusion† 83% Cerebellum 6% Deep midline structures 12% Brain stem 8% Cerebral hemisphere 8% Other 16% Sep 28, 2022 data cutoff. †Includes 8 patients with BRAF duplication or BRAF rearrangement. MAPK, mitogen-activated protein kinase; prior MAPK pathway targeted therapy indicates either prior MEKi and/or prior type I RAFi therapy.

Topline Data from Ongoing Pivotal Phase 2 FIREFLY-1 Trial Day One Biopharmaceuticals Sep 28, 2022 data cutoff. CR, complete response. PR, partial response. SD, stable disease. The primary endpoint of the FIREFLY-1 trial is overall response rate (ORR) by Response Assessment for Neuro-Oncology-High Grade Glioma (RANO-HGG) criteria as assessed by blinded independent central review. In the 69 RANO-evaluable patients: 64% ORR and 91% clinical benefit rate (complete response + partial response/unconfirmed partial response + stable disease) 4% (n=3) confirmed complete responses 59% (n=41) partial responses (31 confirmed and 10 unconfirmed) 28% (n=19) patients with stable disease 86% (n=59) of patients had a BRAF fusion alteration, for which there are no approved systemic therapies, while the remaining 14% (n=10) had a BRAF mutation Safety data, based on 77 treated patients, indicated monotherapy tovorafenib to be generally well-tolerated. The most common side effects reported as related to tovorafenib were change in hair color (75%), increased creatine phosphokinase (64%), anemia (46%), fatigue (42%) and maculopapular rash (42%) 3 patients (3.9%) discontinued treatment due to adverse events, of which 2 (2.6%) were deemed to be related to tovorafenib Among a total of 77 treated patients: Participants were heavily pretreated, with a median of three prior lines of systemic therapy (range: 1-9) The median duration of tovorafenib treatment was 8.4 months, with 77% (n=59) of patients on treatment at the time of the data cutoff Nearly 60% (n=46) of patients had already received at least one prior MAPK inhibitor prior to study participation

~26,000 Estimated Prevalence Incidence and Prevalence of BRAF-altered pLGG in the U.S. Day One Biopharmaceuticals 1. US Census; 2 CBTRUS, Qaddoumi et al 2009, Schreck et al 2019, ClearView Analysis; 3 SEER US complete prevalence counts of patients aged under 25 with Brain and Other Nervous Systems tumors as of January 1, 2017. Estimated annual incidence and estimated prevalence are Day One calculations based on publicly available data. 2020 Estimated Incidence Under 25 US Population1 ~105,000,000 Rate of CNS Tumors (0.00521%)2 ~5,500 Gliomas (63%)2 ~3,500 Low Grade (77%)2 ~2,600 Has Received Drug Tx (58%)2 ~1,500 BRAF Altered (70%)2 ~1,100 2017 Estimated SEER Prevalence Under 25 NA ~130,0003 ~82,000 ~63,000 ~36,000 ~26,000 ~1,100 Estimated Annual Incidence

Day One Biopharmaceuticals FIREFLY-2/LOGGIC Pivotal Phase 3 Trial of Tovorafenib (DAY101) in Newly Diagnosed pLGG

FIREFLY-2/LOGGIC Pivotal Phase 3 Trial of Tovorafenib (DAY101) in Newly Diagnosed pLGG Day One Biopharmaceuticals Primary endpoint: ORR based on RANO-LGG criteria, assessed by blinded independent central review1 The ORR primary analysis is expected to occur ~12 months after the last patient randomized Key secondary endpoints: PFS and DoR by RANO criteria, ORR by RAPNO criteria Other secondary endpoints: changes in neurological and visual function, safety, and tolerability Key exploratory objectives: QoL and health utilization measures Randomized, global, registrational Phase 3 trial of monotherapy tovorafenib (DAY101) vs SoC chemotherapy Eligibility: Patients aged 6 months to <25 years with LGG harboring a RAF alteration and requiring first-line systemic therapy Tovorafenib (DAY101) available as tablets and pediatric-friendly liquid suspension Patients who progress after stopping tovorafenib (DAY101) may be re-challenged Patients who progress in the SoC arm during or post-treatment may cross-over to receive tovorafenib Endpoints Trial Design 1:1 Randomization Tovorafenib, 420mg/m2 QW (not to exceed 600 mg) Investigator's choice of vincristine/carboplatin* or vinblastine Long-term follow-up (48 months) Stratified by Location of tumor Genomic alteration CDKN2A status Infant CHG diagnosis Non-resectable or sub-total resected LGG AND Requiring first-line systemic therapy N ≈ 400 * COG or SIOPe-LGG regimen  Abbreviations: CHG, chiasmatic, hypothalamic glioma; DoR, duration of response; LGG, low-grade glioma; ORR, objective response rate; QoL, quality of life; QW, once weekly; SoC, standard of care. 11Primary endpoint of FIREFLY-2 will be ORR by RANO-LGG (2017) following full approval by FDA on March 16, 2023 of dabrafenib with trametinib in pediatric patients with low-grade glioma with a BRAF V600E mutation who require systemic therapy based on a study with the same primary endpoint.

FIREFLY-2/LOGGIC: Pivotal Phase 3 Study Of Tovorafenib (DAY101) In Newly Diagnosed pLGG Day One Biopharmaceuticals Collaboration between Day One and the LOGGIC consortium, internationally recognized experts in pLGG research Coupled with the LOGGIC-CORE molecular diagnostic program Worked jointly on the study design and discussions with the U.S. and EU regulatory authorities Approximately 100 potential sites (~65 from the LOGGIC consortium) ~65 Sites ~20 Sites ~10 Sites ~5 Sites

Product Candidate Indication Preclinical Phase 1 Phase 2 Phase 3 Recent & Anticipated Milestones Tovorafenib (DAY101) Type II Pan-RAF Inhibitor Relapsed pLGG Frontline pLGG RAF-altered solid tumors2 (monotherapy) Pimasertib MEK 1/2 Inhibitor MAPK-altered solid tumors3 (Combo w/tovorafenib) Our Pipeline *Includes patients ≥12 years of age. 1 FIREFLY-1 Arm 1 expected to support registration. 2 DAY101 adult monotherapy Phase 1 dose escalation and expansion trial previously completed. 3 Pimasertib Phase 1 dose escalation and expansion trial previously completed. pLGG, pediatric low-grade glioma. Tovorafenib and Pimasertib are investigational products. Safety and efficacy have not been established by any health authority. Day One Biopharmaceuticals FDA Breakthrough Therapy Designation for relapsed pLGG FDA Orphan Drug Designation for malignant glioma EC Orphan Designation for glioma FDA Rare Pediatric Disease Designation (PRV Eligible) for pLGG FIREFLY-11 (pivotal) FIREFLY-2 (pivotal) FIRELIGHT-1* FIRELIGHT-1* First patient dosed: March 2023 First patient dosed: November 2021 Poster presented: April 2023 First patient dosed: May 2022 Pre-NDA meeting held: April 19, 2023 Remain in position to initiate NDA submission: Q2 2023 New clinical data presentation planned: June 2023

Phase 2 Study of Monotherapy Tovorafenib (DAY101) in Solid Tumors (FIRELIGHT-1) Day One Biopharmaceuticals Abbreviations: ORR, objective response rate; QW, once weekly ; PO, by mouth; BRAF, B-Raf proto-oncogene. Umbrella master study – DAY101-102 (main protocol) DAY101 and MAPK pathway aberration, Sub-study 1 monotherapy (DAY101-102a), Sub-study 2 MEK combo (DAY101-102b). DAY101 QW until disease progression, intolerable toxicity, withdrawal of consent, or death Primary endpoint: ORR by RECIST version 1.1 for non-CNS solid tumors and RANO-HGG criteria for any CNS tumors Secondary endpoints: safety and additional efficacy parameters Single arm, open-label, global phase 1b/2a trial n = 40 patients (approximately) Eligibility: Patients aged 12 years and older with non-hematologic tumor with an activating BRAF fusion, CRAF/RAF1 fusion, or CRAF/RAF1 amplification Endpoints Trial Design1 Patients with a known BRAF or CRAF/RAF1 fusion, or CRAF/RAF1 amplification Melanoma cohort “Tissue agnostic” cohort Study Drug Administration ≥ 18 years at 600 mg PO QW 12 to <18 years at 420mg/m2 PO QW DAY101 QW until disease progression2 Safety Follow Up

Day One Biopharmaceuticals Preliminary Clinical Activity of Tovorafenib (DAY101) Monotherapy in BRAF Fusion Melanoma Parameter Description/outcome Stage at diagnosis III EGOC status 0 Prior therapies Multiple lymphadenectomies and skin lesion excision surgery Pembrolizumab (11 weeks): Best response: SD Tovorafenib treatment to date in FL-1 102a (melanoma cohort)* 600 mg QW 5 cycles with no dose interruption or modifications due to AEs Antitumor activity results to date* CR (11-week scan)†; confirmed at 16 weeks‡ Safety results to date* TRAEs: Transient rash (G1 and G2) Anemia (G2) TEAE: Neck pain (G1) *Data cutoff Feb 8, 2023. †Out of window per protocol. ‡per RECIST v1.1. AE, adverse event; CR, complete response; ECOG, Eastern Cooperative Oncology Group; FL-1, FIRELIGHT-1; G, grade; QW, once weekly; RECIST, response evaluation criteria in solid tumors; SD, stable disease; TEAE, treatment-emergent adverse event; TRAEs, treatment-related adverse events; y/o, years of age. Patient 1: 53-year-old male with AGK-BRAF fusion non-spitzoid cutaneous melanoma

Day One Biopharmaceuticals Preliminary Clinical Activity of Tovorafenib (DAY101) Monotherapy in BRAF Fusion Melanoma Parameter Description/outcome Stage at diagnosis Unknown EGOC status 1 Prior therapies Radiation Nivolumab (12 mo, adjuvant setting): No best response, disease resected Nivolumab + ipilimumab (3 cycles): Best response: PD after 2 mo Tovorafenib treatment to date in FL-1 102a (melanoma cohort)* 600 mg QW 5 cycles with no dose interruption or modifications due to AEs Antitumor activity results to date* PR (8-week scan); confirmed at 16 weeks† Safety results to date* TRAEs: Rash - maculopapular (G1) Headache (G1) Fatigue (G1) Patient 2: 35-year-old male with TRIM33-BRAF fusion malignant melanoma *Data cutoff Feb 8, 2023. †per RECIST v1.1. AE, adverse event; ECOG, Eastern Cooperative Oncology Group; FL-1, FIRELIGHT-1; G, grade; mo, months; PD, progressive disease; PR, partial response; QW, once weekly; RECIST, response evaluation criteria in solid tumors; TRAEs, treatment-related adverse events; y/o, years of age.

Day One Biopharmaceuticals Preliminary Clinical Activity of Tovorafenib (DAY101) Monotherapy in BRAF Fusion Melanoma Parameter Description/outcome Stage at diagnosis II EGOC status 0 Prior therapies Radiation Pembrolizumab (2 mo): Best response: SD Tovorafenib treatment to date in FL-1 102a (melanoma cohort)* 600 mg QW 3 cycles with no dose interruption or modifications due to AEs Antitumor activity results to date* PR (7-week scan)†,‡; is awaiting a confirmatory scan Safety results to date* TRAEs: Urticaria (G1) Hand-foot syndrome (G1) Patient 3: 71-year-old male with MKRN1-BRAF fusion non-spitzoid cutaneous melanoma *Data cutoff Feb 8, 2023. †In window per protocol. ‡per RECIST v1.1.AE, adverse event; ECOG, Eastern Cooperative Oncology Group; FL-1, FIRELIGHT-1; G, grade; mo, months; PR, partial response; QW, once weekly; RECIST, response evaluation criteria in solid tumors; SD, stable disease; TRAEs, treatment-related adverse events; y/o, years of age.

Activity of Tovorafenib (DAY101) in SNX8:BRAF Fusion Spindle Cell Sarcoma Day One Biopharmaceuticals A male child spindle cell sarcoma, 5-years of age at diagnosis Data cut off: September 30, 2021 Months -30 -24 -18 -12 -6 0 6 Diagnosis BRAF fusion (FISH) CR PD CR Gemcitabine, docetaxel Proton radiotherapy SNX8-BRAF fusion Targeted Therapy Subtotal resection Ifosfamide, doxorubicin, dexrazoxane Tovorafenib Tovorafenib After 2 cycles of tovorafenib Baseline After the first dose of tovorafenib (DAY101), the patient experienced grade 2 rash, which resolved in a day following a dose of diphenhydramine Radiotherapy-related adverse events included hyperpigmentation overlying the spine on the upper back with no skin breaks, and mild dysphagia

Strong Scientific Rationale for Combining Tovorafenib (DAY101) with Additional MAPK Pathway Inhibitors Day One Biopharmaceuticals Type II RAFi + MEKi Type II RAFi + MEKi Type II RAFi + SHP2i MEKi or Type II RAFi + SHP2i MEKi or KRAS - G12Ci or Potential combinations BRAF non-V600 BRAF or CRAF fusion KRAS or NRAS mutant NF1 LOF Rationale Non V600 BRAF dimers are effectively inhibited by type II, but not type I, RAFi BRAF fusion dimers are effectively inhibited by type II, but not type I RAFi Targeting multiple nodes of MAPK pathway will drive deeper and more durable response Targeting multiple pathways will drive deeper response Signaling pathways Proliferation, survival MEK ERK Proliferation, survival RAS/RAF/ MEK/ERK PI3K/ mTOR Proliferation, survival RAF/ MEK/ERK RAL PI3K/m TOR Proliferation, survival MEK ERK

Day One Biopharmaceuticals Pimasertib MEK1/2 Inhibitor

Pimasertib: Investigational Allosteric MEK1/2 Inhibitor with Demonstrated Activity in MAPK-driven Solid Tumors Day One Biopharmaceuticals Pimasertib is an investigational orally-bioavailable, selective, non-competitive MEK1/2 inhibitor in-licensed from Merck KGaA in February 2021 Extensive non-clinical and clinical development work through Phase 2, including a solid tumor trial in Japan and combinations with other MOAs Main AEs typical for all in-class allosteric MEK inhibitors (GI, CPK elevation, skin rash, visual disturbances) Nearly three-fold higher CNS penetration than other MEKi inhibitors (trametinib or selumetinib) Pimasertib showed monotherapy clinical activity, including an improvement in median PFS versus dacarbazine in NRAS mutant melanoma Combination with tovorafenib (DAY101) and other targeted therapies may unlock the full value of pimasertib in advanced solid tumors Sources: Pimasertib Investigator Brochure, v12, 2019; de Gooijer et al., Int J Cancer, 2018; Shaw et al., AACR LB-456, 2012; Lebbe et al., Cancers, 2020.

Vertical MAPK Pathway Inhibition with Tovorafenib (DAY101) and Pimasertib May Unlock Potential Synergy for Adult Solid Tumors Day One Biopharmaceuticals Pan-RAFi + MEKi is synergistic in BRAF fusion melanoma PDX model ex vivo (internal data) Sensitivity of KRAS Q61 mutant cells to pimasertib is enhanced when cells are treated with the type II BRAF inhibitor BGB-283 (Yuan et al., Mol Onc 2020) Tovorafenib (DAY101) + MEK inhibitor is synergistic in KRAS G12C and Q61 mutant tumor cells (Venetsanakos et al., 2021 AACR poster presentation) B C Type II RAFi + MEKi KRAS or NRAS mutant Targeting multiple nodes of MAPK pathway will drive deeper and more durable response Proliferation, survival RAF/ MEK/ERK RAL PI3K/m TOR A Type II RAFi + MEKI BRAF non-V600 mutant Non V600 BRAF dimers are effectively inhibited by type II RAFi , but not type I BRAFi Proliferation, survival MEK ERK

Tovorafenib (DAY101) / Pimasertib Combination to be Evaluated in Solid Tumors (FIRELIGHT-1) Day One Biopharmaceuticals Abbreviations: BOIN, Bayesian Optimal Interval Design; BRAF, B-Raf proto-oncogene, serine/threonine kinase; MAPK, mitogen-activated protein kinase; NRAS, neuroblastoma rat sarcoma viral oncogene. Umbrella master study – DAY101-102 (main protocol) DAY101 and MAPK pathway aberration, Sub-study 1 monotherapy (DAY101-102a), Sub-study 2 MEK combo (DAY101-102b). Intend to open U.S. and ex-U.S. clinical sties. 3DAY101 + Pimasertib until disease progression, intolerable toxicity, withdrawal of consent, or death Phase 1b: PK, PD and Safety, MTD/RP2D Phase 2: Efficacy (ORR, DOR) Combination dose escalation, global phase 1b/2 trial2 Phase 1b, BOIN (adaptive), n = 10/cohort (approximately) Phase 2, Simon 2-stage, n = 25/cohort (approximately) Eligibility: Patients aged 12 years and older, dose escalation will be performed in advanced solid tumor patients with any MAPK alteration. Expansion cohorts will focus on indications with a potential path to accelerated approval Endpoints Trial Design1 Pre-identified patients with advanced solid tumors and available clinical molecular profiling information. Phase 1b Tumors with MAPK pathway alterations *Additional biomarker-selected cohorts may be pursued based on developing data Phase 2* NRASmut Selected tumors BRAF fusion selected tumors BRAF Class 1 (non-E/K) and Class 2 mutant tumors DAY101 + Pimasertib until disease progression3 Safety Follow Up

Day One Biopharmaceuticals Summary

Financial Summary: DAWN Day One Biopharmaceuticals $ Millions Three Months Ended3/31/23 Three Months Ended3/31/22 R&D Expense $27.8 $15.0 G&A Expense $18.0 $12.7 Net Loss $42.4 $27.7 FIREFLY-1: Pivotal Phase 2 clinical trial of tovorafenib (DAY101) Pre-NDA meeting held on April 19, 2023 Remain in position to initiate the NDA submission in Q2 2023 Presentation of new clinical data1 in oral presentation at ASCO in June 2023 FIREFLY-2/LOGGIC: Pivotal Phase 3 clinical trial of tovorafenib (DAY101) in newly diagnosed pLGG First patient dosed in March 2023 Cash, cash equivalents and short-term investments as of March 31, 2023: $318.2 million (no debt) 73.6 million shares of common stock outstanding as of April 25, 2023 Projected cash runway into 2025 All financial and share information is unaudited. 1NDA data set will include analysis of primary (ORR by RANO-HGG) and secondary (ORR by RAPNO, PFS) efficacy endpoints, safety, and exploratory analyses (including ORR by RANO-LGG).

Next Steps Day One Biopharmaceuticals FIREFLY-1 Pre-NDA meeting held on April 19, 2023 Remain in position to initiate the NDA submission in Q2 2023 Presentation of new clinical data in oral presentation at ASCO conference FIREFLY-2 Advance tovorafenib as a front-line therapy for patients newly diagnosed with pLGG Commercial Continue investment in market and launch preparation activities Topline data from pivotal Phase 2 FIREFLY-1 trial demonstrating meaningful responses with tovorafenib in recurrent or progressive pLGG Overall response rate of 64% and clinical benefit rate of 91% in 69 heavily-pretreated, RANO-HGG evaluable patients Median duration of 8.4 months on therapy as of data cut, with 77% of patients remaining on treatment Safety data, based on the 77 treated patients, indicated monotherapy tovorafenib to be generally well-tolerated Data cut off as of September 28, 2022. Overall response rate of 64% in 69 heavily-pretreated, RANO-evaluable patients includes confirmed and unconfirmed CR and PR. Clinical benefit rate (complete response + partial response/unconfirmed partial response + stable disease). ORR by RANO-HGG Business Development Further investment in business development activities to expand our multiple asset portfolio FIRELIGHT-1 Evaluate tovorafenib in combination and as monotherapy in adolescent and adult populations Monotherapy abstract presented at EADO in April 2023

Appendix Day One Biopharmaceuticals

FIREFLY-1 Baseline Characteristics Day One Biopharmaceuticals Characteristic Arm 1 (N=25) Median age, years (range) 8 (3-18) Sex, n (%) Male Female 13 (52) 12 (48) Race, n (%) Black or African American Asian White Other* 1 (4) 2 (8) 15 (60) 7 (28) Karnofsky/Lansky performance status, n (%) 50-70 80-100 1 (4) 24 (96) Number of lines of prior therapy Median (range) 1, n (%) 2, n (%) ≥3, n (%) 3 (1-9) 5 (20) 6 (24) 14 (56) Prior MAPK pathway targeted therapy, n (%) Yes No 18 (72) 7 (28) Apr 14, 2022 data cutoff; *Includes 4 patients with race not specified. †Includes 2 patients with BRAF duplication and 1 with BRAF rearrangement per fluorescence in situ hybridization. MAPK, mitogen-activated protein kinase; prior MAPK pathway targeted therapy indicates either prior MEKi and/or prior type I RAFi therapy. BRAF alteration (n=25) Location (n=25) Optic pathway 52% BRAF V600E 16% BRAF fusion† 84% Cerebellum 4% Deep midline structures 12% Brain stem 8% Hypothalamus 8% Other 16%

Tumor Response To Tovorafenib (DAY101) For All Patients With RANO-HGG Evaluable Lesions (n=22)* Day One Biopharmaceuticals Response (IRC) RANO-HGG Evaluable N=22* ORR (95% CI) 64% (41-83) BRAF fusion (n=20) 60% BRAF V600E (n=2) 100% CBR# 91% Best overall response PR (13/22) 59% uPR (1/22) 5% SD (6/22) 27% Apr 14, 2022 data cutoff. Total % of response maybe may be different than the sum of the individual overall response due to rounding. *3/25 patients lacked evaluable lesions per RANO criteria based on IRC evaluation. †Progressive disease due to presence of new lesions. #patients with best overall response of CR, PR/uPR and SD. CBR, clinical benefit rate; IRC, independent radiological review committee; ORR, overall response rate; MAPK, mitogen-activated protein kinase; PR, partial response; SD, stable disease; uPR, unconfirmed partial response.

Duration of Tovorafenib (DAY101) Therapy For All Patients with RANO-HGG Evaluable Lesions (n=22) Day One Biopharmaceuticals Apr 14, 2022 data cutoff. 17/22 patients remain on therapy All responders remain on treatment

Individual Patient Tumor Change From Baseline(n=22 RANO-HGG Evaluable By Blinded Independent Central Review) Day One Biopharmaceuticals PR PD SD CR Percent change from baseline (%) Months Apr 14, 2022 data cutoff. RANO PD = ≥+25% change from baseline; RANO SD = <+24% to > -50% change from baseline; RANO PR = ≤ -50% change from baseline; RANO CR = -100% change from baseline.

Tovorafenib (DAY101) Safety Data For the First 25 Enrolled Patients (TEAEs ≥25% Any Grade) Day One Biopharmaceuticals Preferred term, n (%) Treatment-emergent AEs Treatment-related AEs Any grade Grade ≥3 Any grade Grade ≥3 Blood creatine phosphokinase increased 20 (80) 2 (8) 18 (72) 2 (8) Hair color changes 17 (68) - 17 (68) - Anemia 14 (56) 3 (12) 10 (40) 2 (8) Aspartate aminotransferase increased 14 (56) - 12 (48) - Vomiting 14 (56) 2 (8) 6 (24) 1 (4) Rash* 13 (52) 3 (12) 13 (52) 3 (12) Blood lactate dehydrogenase increased 12 (48) - 9 (36) - Headache 10 40) - 3 (12) - Dry skin 9 (36) - 7 (28) - Epistaxis 9 (36) - 4 (16) - Constipation 8 (32) - 5 (20) - Hypocalcemia 8 (32) - 6 (24) - Nausea 8 (32) - 3 (12) - Alanine aminotransferase increased 7 (28) 1 (4) 4 (16) 1 (4) Fatigue 7 (28) - 7 (28) - Most treatment-emergent AEs were grade 1 or 2 (96%) Other important treatment-emergent AEs included: Decreased weight (24%) Decreased appetite (16%) Hyponatremia (16%) 7 patients (28%) required dose modifications due to treatment-related AEs No patient discontinued treatment due to AEs Apr 14, 2022 data cutoff. AE, adverse event. TEAE, treatment-emergent adverse event. *Includes maculopapular and erythematous rash

Key Takeaways Day One Biopharmaceuticals Encouraging initial efficacy data from FIREFLY-1 for pediatric patients with recurrent or progressive LGG harboring BRAF fusion or BRAF V600 mutation, for whom there is no standard-of-care and no approved agents for the majority of patients 64% ORR and 91% clinical benefit rate (partial response/unconfirmed partial response + stable disease) in the 22 RANO-HGG evaluable patients: 14 partial responses (13 confirmed responses and 1 unconfirmed response) 6 patients with stable disease All patients with stable disease (n=6) were noted to have tumor shrinkage, ranging between 19% and 43% Responses were observed in patients with both BRAF fusions and BRAF V600E mutations who received prior MAPK-targeted therapy The median-time-to-response was 2.8 months A heavily-pretreated population, with a median of 3 prior lines of therapy (range: 1-9) All patients who responded remain on therapy (n=14) and no patients have discontinued treatment due to treatment-related adverse events Initial safety data, based on the first 25 patients, indicated monotherapy tovorafenib (DAY101) to be generally well-tolerated Majority of AEs were grade 1 or 2; most common treatment-related AEs were CPK elevation, rash, and hair color changes Treatment-related AEs of grade 3 or greater occurred in nine patients (36%) Plan to present additional initial study results from FIREFLY-1 at the Society for Neuro-Oncology (SNO) annual meeting Topline results from the full registrational cohort (n=~60) of FIREFLY-1 expected to be available 1Q 2023, with NDA submission planned for Q2 2023 Early results from FIREFLY-1 support plan to evaluate tovorafenib (DAY101) in parallel with a pivotal Phase 3 frontline pLGG study (FIREFLY-2) Primary endpoint of ORR based on RANO-LGG (2017)1 criteria, assessed by blinded independent central review 11Primary endpoint of FIREFLY-2 will be ORR by RANO-LGG (2017) following full approval by FDA on March 16, 2023 of dabrafenib with trametinib in pediatric patients with low-grade glioma with a BRAF V600E mutation who require systemic therapy based on a study with the same primary endpoint.

Case Study: Activity Of Tovorafenib (DAY101) In KIAA1549-BRAF Fusion Optic Pathway Glioma Day One Biopharmaceuticals Apr 14, 2022 data cutoff. Tovorafenib is an investigational agent. Safety and efficacy have not been established by any health authority. PR (-58%) and improvement in vision reported at cycle 3 AEs included grade 3 erythematous rash requiring dose interruption and dose reduction (400 mg QW to 300 mg QW in cycle 1), and grade 2 eczema and maculopapular rash Patient continues to receive weekly tovorafenib PR 1L: Vincristine/carboplatin Best response SD Months -36 -24 -18 -12 -6 0 6 -30 Diagnostic biopsy 2L: Trametinib Best response PD 3L: Tovorafenib A 7-years-old female child with an optic pathway glioma, with very poor vision, entropion, folliculitis, eczema, mouth ulceration and xerosis Baseline After 6 cycles Tumor kinetics (RANO) Tovorafenib

Case Study: Activity Of Tovorafenib (DAY101) In KIAA1549-BRAF Fusion Posterior Fossa Pilocytic Astrocytoma Day One Biopharmaceuticals Apr 14, 2022 data cutoff. Tovorafenib is an investigational agent. Safety and efficacy have not been established by any health authority. Baseline After 6 cycles After 3 cycles After 9 cycles Tumor kinetics PR (-69%) at cycle 3 with 500 mg QW tovorafenib, with a deepening of response (80% and 91% in cycles 6 and 9, respectively) over time AEs included grade 2 decrease in neutrophil count, pustular rash, and upper respiratory infection Patient continues to receive weekly tovorafenib Suboccipital craniotomies 2L: Carboplatin/vincristine Best response PR PR 3L: Vinblastine Best response SD Months -84 -48 -36 -24 -12 0 12 -72 4L: Trametinib Best response SD 5L: Tovorafenib -60 1L: Carboplatin Best response PD An 8-years-old female child with a posterior fossa pilocytic astrocytoma, eczema, nausea and constipation Tovorafenib

Case Study: Activity Of Tovorafenib (DAY101) In BRAF V600E Mutation Deep Midline Astrocytoma Day One Biopharmaceuticals Apr 14, 2022 data cutoff. Tovorafenib is an investigational agent. Safety and efficacy have not been established by any health authority. Baseline After 6 cycles After 3 cycles Tumor kinetics PR (-74%) at cycle 3, with a deepening of response (-94%) at cycle 6 AEs included grade 3 maculopapular rash and increased CPK, requiring drug interruption and dose reduction (500 mg QW to 400 mg QW in cycle 1) Tovorafenib dose was re-escalated back to 500 mg QW in cycle 4; patient continues on treatment PR Months -48 -36 -24 -12 0 12 -60 2L: Dabrafenib Best response PR 3L: Tovorafenib 1L: Carboplatin/vincristine Best response PR Subtotal resection A 9-year-old female child with deep midline BRAF V600E-mutant astrocytoma with precocious puberty Tovorafenib

Results from Independent Radiology Review of PNOC014 Day One Biopharmaceuticals RAPNO: Response assessment for pediatric neuro-oncology (exploratory) RANO-HGG: Response assessment for neuro-oncology-high grade glioma Volumetric image analysis (exploratory) Best response from baseline Date of data cutoff: 02 JAN 2020 Wright K et. al. Neuro Oncology Abstract CTNI-19. 2020

Multiple Rapid, Deep and Durable Responses Observed following Initiation of Tovorafenib (DAY101) Treatment of pLGG Patients in PNOC014 Day One Biopharmaceuticals Date of data cutoff: 02 JAN 2020 Adapted from Wright K et. al. Neuro Oncology Abstract CTNI-19. 2020Fangusaro J et al. Lancet Oncol 2019

Drug-related Adverse Events Observed for Tovorafenib (DAY101) in PNOC014 Showed Favorable Safety and Tolerability Profile in pLGG Day One Biopharmaceuticals Date of DAY101 data cutoff: 02 JAN 2020; Wright K et. al. Neuro Oncology Abstract CTNI-19. 2020; Fangusaro J et al. Lancet Oncol 2019 Toxicities Grade 1-2 Grade 3 Grade 4 Anemia 6 (67%) Hypophosphatemia 4 (44%) Fatigue 5 (55%) Rash 8 (89%) Achromotrichia 7 (78%) Pruritis 6 (67%) Photosensitivity 1 (11%) Nevus 7 (78%) Alopecia 3 (34%) Epistaxis 2 (22%) Dry skin 3 (34%) Myalgias/arthralgias 3 (34%) Anorexia 2 (22%) Cheilitis 3 (34%) Hypermagnesemia 1 (11%) Bleeding gums 1 (11%) Increased AST 4 (44%) Nausea/vomiting 3 (33%) CPK elevation 1 (11%) Weight loss 2 (22%) DAY101 AE summary Most common toxicity: skin AEs reversible and all manageable Single, reversible Grade 3 event No Grade 4 AEs No dose reductions (vs. 40% of patients on selumetinib montherapy required dose reductions) Drug-related AEs for Tovorafenib (DAY101) Toxicities Grade 1-2 Grade 3 Grade 4 Increased ALT 20 (40%) 1 (2%) CPK elevation 34 (68%) 5 (10%) Diarrhea 27 (54%) 2 (4%) Decreased ejection fraction 19 (38%) 1 (2%) Gastric haemorrhage 1 (2%) Headache 14 (28%) 1 (2%) Decreased lymphocyte count 19 (38%) 1 (2%) Neutropenia 14 (28%) 3 (6%) Paronychia 19 (38%) 3 (6%) Rash (acneiform) 29 (58%) 2 (4%) Rash (maculopapular) 26 (52%) 5 (10%) Skin infection 7 (14%) 1 (2%) Tooth infection 1 (2%) Weight gain 5 (10%) 1 (2%) Vomiting 22 (44%) Nausea 21 (42%) Increased AST 25 (50%) Anemia 28 (56%) Pruritis 10 (20%) Dyspnea 30 (60%) Drug-related AEs for selumetinib