2seventy R&D Deep Dive: ASGCT and Beyond May 19, 2023

Cautionary note regarding forward-looking statements These slides and the accompanying oral presentation may contain “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. These statements include, but are not limited to: statements about our plans, strategies, timelines and expectations with respect to the research, development, manufacture or sale of our product candidates, including the design, initiation, enrollment, completion and results of pre-clinical and clinical studies; timelines for the results of ongoing and planned clinical trials for our product candidates and for ABECMA (ide-cel) in additional indications; the timing or likelihood of regulatory filings and acceptances and approvals thereof; expectations as to the market size for ABECMA and any other approved product we may successfully develop; the progress and results of our commercialization of ABECMA, including our goal of increasing manufacturing capacity and improving the manufacturing process and the number of patients that are expected to be treated with ABECMA in the commercial setting and potential late line global revenue for ABECMA; anticipated revenues resulting from sales of ABECMA; statements about the efficacy and perceived therapeutic benefits of our product candidates and the potential indications and market opportunities therefor; statements about the strategic plans for 2seventy bio and potential corporate development opportunities, including manufacturing expectations and benefits received from collaborations; statements about our ability execute our strategic priorities; and expectations regarding our use of capital, expenses and other future financial results, including our net cash spend, cash runway and U.S. net revenue for ABECMA in 2023 and beyond. 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Agenda and Vision 3 Nick Leschly, chief kairos officer

The sole mission of 2seventy is to “unleash the curative potential of the T cell” Our experience in drug development and deep execution capabilities in cell therapy allow us to design & deliver multi-layered, multi-modality T cell-based solutions that have the potential to address and overcome the immunologically evasive and suppressive properties of tumors. TIME GEEKS 4

2seventy – At a Glance ABECMA Commercial ABECMA Development BMS 50/50 US $297M 2022 / $470-570M 2023* Potential $2-3B US Peak 3rd Line K3 & FDA Submission Initiation of Front Line K9 ABECMA® Capacity Expansion Predictable Delivery ABECMA profitable in 2023** Cash runway into 2026** 425 employees Unleash Curative Potential of T cells TIME GEEKS Suspension LVV Mftg In-House Clinical DP Mftg (270-MPH) Next Gen Mftg Improvements Next Gen T cell Engineering Toolkit & Clinical POC Insight Engine Clinical Heme B-NHL & AML Clinical Solid MUC & More (REGN) Horizon Dev In-Vivo Editing (Novo) In-Vivo CARs *Anticipated revenue, US topline revenue, profit and loss shared 50/50 with BMS **Projected, based on current operating plan and anticipated revenue People & Culture & Experience 5 B-NHL: B-cell non-Hodgkin lymphoma AML: acute myeloid leukemia

Agenda TOPIC SPEAKER Corporate Strategy and Vision ABECMA Clinical, Operational and Financial Progress Advances Across Pipeline and Internal DP Manufacturing AML Clinical and Preclinical Developments Ex Vivo and In Vivo Gene Editing Applications Wrap-Up and Questions Nick Leschly, chief kairos officer Chip Baird, chief financial officer Philip Gregory, D.Phil., chief scientific officer Steve Bernstein, M.D., chief medical officer Steve Shamah, Ph.D., SVP, oncology research Mike Certo, Ph.D., VP, head of genome editing All 6

ABECMA and R&D Financial Overview 7 Chip Baird, chief financial officer

ABECMA and Financial Overview Four key takeaways… ABECMA continues to achieve strong revenue growth ABECMA is cash flow positive with improving margins ABECMA has an attractive and long-term commercial trajectory 2seventy is efficient with its R&D investment 1 2 3 4 8

$0 $25 $50 $75 $100 $125 $150 $175 $200 $225 $250 $275 $300 $325 $350 $375 $400 W W S a le s ( in m ill io n s ) Quarter(s) Following Approval WW Sales of CAR T Therapies, by Quarter of Launch Kymriah (Novartis) Yescarta (Gilead) Tecartus (Gilead) Breyanzi (BMS) ABECMA (BMS/2seventy) Carvykti (J&J/Legend) ABECMA launch growth trajectory driven by efficacy profile, strong patient demand, and manufacturing step-ups Average Annual Growth Rate Year 4 29% Year 3 49% Year 2 94% Year 5 33% Class CAGR for years 3-5: 37% 1 Strong revenue growth Source: Company filings 9 ABECMA

Strong start to 2023 for ABECMA May 2023 update •Cash flow positive in 1Q23 •On track to achieve upper end of $470-570M* revenue guidance •Second aLVV suite approved; on track for sLVV approval in 1H24 •Successful DP step-up complete; additional step- ups on track for 2023 •$200-300M of operating income expected for the 2024-25 timeframe** 10 2 Growing cash flow

Real-world MM treatment decisions are practical and patient-driven Efficacy Potential for deep and durable response Safety Predictable and manageable side-effect profile Real World Experience Physician real-world experience with the product Availability Slot availability in the relevant time frame Turn-around time Speed to manufacturing and deliver patient cells Out of Spec Risk Percentage of time cells are out of spec Select ABECMA Launch Metrics Through Dec 2022 • Over 1,100 US commercial patients treated since launch • ~70 treatment centers online in the U.S.; additional centers planned in 2023 • 85-90% average in-spec manufacturing success since launch • ~30-day average turn-around-time End-to-End Patient and Provider Experience 11 3 Durable commercial trajectory

Assuming capacity growth in-line with CD-19 experience, more than half of eligible patients will not have access to a CAR T in 2025 12 2023 (5L+) 2024 (3L+) 2025 (3L+) * Illustrative US Multiple Myeloma CAR T Capacity Growth Scenario 37% CAR T capacity growth 37% CAR T capacity growth 2026-30 (label expansion) 3 Assumptions and Methodology 30,000 US MM patients 2023 patients treated based on analyst estimates for commercially approved BCMA CAR Ts 2024 & 2025 patients treated based on 37% annual growth from 2023 levels Assumes commercially approved BCMA CAR Ts achieve 3L+ label by end of 2023 *Each figure = ~1,000 patients = RRMM patients treated with CAR T = not eligible for CAR T= RRMM patients eligible for CAR T but cannot be served due to capacity constraint Durable commercial trajectory

Even with 100% annual growth in commercial capacity, 50% of eligible patients will not be able to receive a CAR T in 2025 13 100% CAR T capacity growth 100% CAR T capacity growth * *Each figure = ~1,000 patients = RRMM patients treated with CAR T = not eligible for CAR T= RRMM patients eligible for CAR T but cannot be served due to capacity constraint 3 Illustrative US Multiple Myeloma CAR T Capacity Growth Scenario Assumptions and Methodology 30,000 US MM patients 2023 patients treated based on analyst estimates for commercially approved BCMA CAR Ts 2024 & 2025 patients treated based on 100% annual growth from 2023 levels Assumes commercially approved BCMA CAR Ts achieve 3L+ label by end of 2023 2023 (5L+) 2024 (3L+) 2025 (3L+) 2026-30 (label expansion) Durable commercial trajectory

R&D Spend in Context Disciplined investment across the portfolio to drive innovation 1Q23 GAAP R&D Expense, as reported ($M) 100 85 80 18 67 66 62 13 38 33 50 44 TSVT gross TSVT net of collab. reimbursements SANA ATRAFATEALLO PSTXLEGNBEAM ACLX Ø 66 68 57 R&D as reported TSVT Net R&D (non-Abecma) TSVT Gross R&D (non-Abecma) Abecma R&D 4 R&D supported by $11M of funding support from BMS, REGN, and Novo Gated approach to capital allocation Win-or-go-home study design ABECMA commercial cash flows fund increasing share of R&D over time Efficient R&D Investment 14

Advances Across Pipeline and Internal DP Manufacturing 15 Philip Gregory, D.Phil., chief scientific officer

2seventy bio’s R&D philosophy Accelerate with Industry Leading Partnerships Identify the Key Problem Layer Innovations Learn Fast in the Clinic Understand tumor resistance Explore new biology Focus on the hard problem Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions 270-MPH (Internal DP Mfg) Turbo-charge Clin. Translation Operational flex & efficiency 16

BINDER TECHNOLOGIES ADVANCED SCREENING PROTEIN ENGINEERING TUMOR IMMUNOLOGY CAPABILITIES CAR TCR CCR DARIC megaTAL PLATFORM PROTEIN ENGINEERING CELL/DRUG PHARMACOLOGY MOLECULAR IMMUNOLOGY ENHANCED COSTIM REGULATED megaTAL PLATFORM PROTEIN ENGINEERING IN VIVO MODELING MOLECULAR IMMUNOLOGY TME BIOLOGICS EXHAUSTION IN VIVO PHARMACOLOGY PROCESS INNOVATION CELLULAR IMMUNOLOGY APPLIED IMMUNOLOGY gd T CELLS NK CELLS CELL CHASSIS POTENCY ENHANCEMENT RECEPTOR SIGNALING TARGETING ARCHITECTURE ab T CELLS METABOLISM PROCESS WORLD CLASS mRNA & LVV VECTOROLOGY / MANUFACTURING DIMENSION TECHNOLOGY Our Innovation Ecosystem ~10 years in the making Layer Innovations Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions IL15* 17

BINDER TECHNOLOGIES ADVANCED SCREENING PROTEIN ENGINEERING TUMOR IMMUNOLOGY CAPABILITIES CAR TCR CCR DARIC megaTAL PLATFORM PROTEIN ENGINEERING CELL/DRUG PHARMACOLOGY MOLECULAR IMMUNOLOGY ENHANCED COSTIM REGULATED megaTAL PLATFORM PROTEIN ENGINEERING IN VIVO MODELING MOLECULAR IMMUNOLOGY TME BIOLOGICS EXHAUSTION IN VIVO PHARMACOLOGY PROCESS INNOVATION CELLULAR IMMUNOLOGY APPLIED IMMUNOLOGY gd T CELLS NK CELLS CELL CHASSIS POTENCY ENHANCEMENT RECEPTOR SIGNALING TARGETING ARCHITECTURE ab T CELLS METABOLISM PROCESS WORLD CLASS mRNA & LVV VECTOROLOGY / MANUFACTURING DIMENSION Our Innovation Ecosystem ~10 years in the making Layer Innovations Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions IL15* Solid Tumor Example: MUC16 18 TECHNOLOGY

BINDER TECHNOLOGIES ADVANCED SCREENING PROTEIN ENGINEERING TUMOR IMMUNOLOGY CAPABILITIES CAR TCR CCR DARIC megaTAL PLATFORM PROTEIN ENGINEERING CELL/DRUG PHARMACOLOGY MOLECULAR IMMUNOLOGY ENHANCED COSTIM REGULATED megaTAL PLATFORM PROTEIN ENGINEERING IN VIVO MODELING MOLECULAR IMMUNOLOGY TME BIOLOGICS EXHAUSTION IN VIVO PHARMACOLOGY PROCESS INNOVATION CELLULAR IMMUNOLOGY APPLIED IMMUNOLOGY gd T CELLS NK CELLS CELL CHASSIS POTENCY ENHANCEMENT RECEPTOR SIGNALING TARGETING ARCHITECTURE ab T CELLS METABOLISM PROCESS WORLD CLASS mRNA & LVV VECTOROLOGY / MANUFACTURING DIMENSION Our Innovation Ecosystem ~10 years in the making Layer Innovations Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions IL15* Example: SC-DARIC33 19 TECHNOLOGY

BINDER TECHNOLOGIES ADVANCED SCREENING PROTEIN ENGINEERING TUMOR IMMUNOLOGY CAPABILITIES CAR TCR CCR DARIC megaTAL PLATFORM PROTEIN ENGINEERING CELL/DRUG PHARMACOLOGY MOLECULAR IMMUNOLOGY ENHANCED COSTIM REGULATED megaTAL PLATFORM PROTEIN ENGINEERING IN VIVO MODELING MOLECULAR IMMUNOLOGY TME BIOLOGICS EXHAUSTION IN VIVO PHARMACOLOGY PROCESS INNOVATION CELLULAR IMMUNOLOGY APPLIED IMMUNOLOGY gd T CELLS NK CELLS CELL CHASSIS POTENCY ENHANCEMENT RECEPTOR SIGNALING TARGETING ARCHITECTURE ab T CELLS METABOLISM PROCESS WORLD CLASS mRNA & LVV VECTOROLOGY / MANUFACTURING DIMENSION Our Innovation Ecosystem ~10 years in the making Layer Innovations Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions IL15* Example: NG-AML – 27T32 DUAL RESET 20 TECHNOLOGY

2seventy bio’s NEW in-house manufacturing facility (270-MPH) The heart of our translational cell therapy engine - Enable Fully Integrated Translational Cell Therapy Platform Enables manufacture and release of drug product for multiple Phase I clinical trials Co-located @ 60 Binney with research, PD and analytics Anticipated ~300 patients/year capacity Accelerates product development learnings and iteration Enhance Clinical Study Flexibility, Speed and Efficiency Provides clinical slot flexibility and faster patient data turnaround/analysis Shortens DP turnaround time and enables efficient monitoring/trouble shooting Significant costs savings through Phase 1 compared to CDMO costs Facility qualification nearing completion and we expect to be fully GMP operational by summer 2023 21

INDICATION [DRUG] TARGET TECHNOLOGY DISCOVERY STAGE R&D IND-ENABLING PRECLINICAL STUDIES CLINICAL STUDIES APPROVED PRODUCTS Multiple Myeloma [ABECMA] BCMA CAR T cell Multiple Myeloma [ABECMA] BCMA CAR T cell AML-Pediatric [SC-DARIC33] CD33 Drug-Regulated; CAR T cell (DARIC) B-NHL [bbT369] CD79a CD20 CBLB Edit Dual-Targeted CAR T cell Signal Enhanced Gene Edited Ovarian Cancer MUC16 CAR T cell Pharmacologic Enhancements Solid Tumors MAGE-A4 TCR T cell Potency Enhanced AML-Adult [SC-DARIC33 Next-Gen] CD33 CLL-1 Drug-Regulated RESET T cell Dual-Targeted Potency Enhanced Solid Tumors Multiple CAR / TCR T cell Potency Enhanced Multiple Myeloma Multiple Multi-Targeted CAR T cell Potency Enhanced Additional Indications Undisclosed Multiple Patients Enrolling; Update mid 2023 TSVT Owned TSVT Owned Multiple TSVT Owned; Plus Novo Nordisk Collab. TSVT Owned; SCRI Collaboration REGN Collaboration BMS Partnership; Approved in 5L+ TSVT Owned Multiple TSVT Owned; Plus Regeneron Collab. BMS Partnership; Earlier Line Studies 3L+ potential approval 2023 NDMM study initiation 2023 REGN/JW Collaboration Innovative cell therapy candidates targeting broad potential indications 22 IND potential EOY 2023 Product engine generating ~1+ INDs per year Patients Enrolling; Update in 2023 IIT potential EOY 2023 (JW / China) Collaboration program TSVT-owned program *Investigational New Drug application – IND; Investigator Initiated Trial – IIT; Newly Diagnosed Multiple Myeloma – NDMM

INDICATION [DRUG] TARGET TECHNOLOGY DISCOVERY STAGE R&D IND-ENABLING PRECLINICAL STUDIES CLINICAL STUDIES APPROVED PRODUCTS Multiple Myeloma [ABECMA] BCMA CAR T cell Multiple Myeloma [ABECMA] BCMA CAR T cell AML-Pediatric [SC-DARIC33] CD33 Drug-Regulated; CAR T cell (DARIC) B-NHL [bbT369] CD79a CD20 CBLB Edit Dual-Targeted CAR T cell Signal Enhanced Gene Edited Ovarian Cancer MUC16 CAR T cell Pharmacologic Enhancements Solid Tumors MAGE-A4 TCR T cell Potency Enhanced AML-Adult [SC-DARIC33 Next-Gen] CD33 CLL-1 Drug-Regulated RESET T cell Dual-Targeted Potency Enhanced Solid Tumors Multiple CAR / TCR T cell Potency Enhanced Multiple Myeloma Multiple Multi-Targeted CAR T cell Potency Enhanced Additional Indications Undisclosed Multiple Patients Enrolling; Update mid 2023 TSVT Owned TSVT Owned Multiple TSVT Owned; Plus Novo Nordisk Collab. TSVT Owned; SCRI Collaboration REGN Collaboration BMS Partnership; Approved in 5L+ TSVT Owned Multiple TSVT Owned; Plus Regeneron Collab. BMS Partnership; Earlier Line Studies 3L+ potential approval 2023 NDMM study initiation 2023 REGN/JW Collaboration MUC16 and MAGE-A4 solid tumor programs are on track for 2023 milestones 23 Product engine generating ~1+ INDs per year Patients Enrolling; Update in 2023 *Investigational New Drug application – IND; Investigator Initiated Trial – IIT; Newly Diagnosed Multiple Myeloma – NDMM Collaboration program TSVT-owned program IND potential EOY 2023 IIT potential EOY 2023 (JW / China)

INDICATION [DRUG] TARGET TECHNOLOGY DISCOVERY STAGE R&D IND-ENABLING PRECLINICAL STUDIES CLINICAL STUDIES APPROVED PRODUCTS Multiple Myeloma [ABECMA] BCMA CAR T cell Multiple Myeloma [ABECMA] BCMA CAR T cell AML-Pediatric [SC-DARIC33] CD33 Drug-Regulated; CAR T cell (DARIC) B-NHL [bbT369] CD79a CD20 CBLB Edit Dual-Targeted CAR T cell Signal Enhanced Gene Edited Ovarian Cancer MUC16 CAR T cell Pharmacologic Enhancements Solid Tumors MAGE-A4 TCR T cell Potency Enhanced AML-Adult [SC-DARIC33 Next-Gen] CD33 CLL-1 Drug-Regulated RESET T cell Dual-Targeted Potency Enhanced Solid Tumors Multiple CAR / TCR T cell Potency Enhanced Multiple Myeloma Multiple Multi-Targeted CAR T cell Potency Enhanced Additional Indications Undisclosed Multiple Patients Enrolling; Update mid 2023 TSVT Owned TSVT Owned Multiple TSVT Owned; Plus Novo Nordisk Collab. TSVT Owned; SCRI Collaboration REGN Collaboration BMS Partnership; Approved in 5L+ TSVT Owned Multiple TSVT Owned; Plus Regeneron Collab. BMS Partnership; Earlier Line Studies 3L+ potential approval 2023 NDMM study initiation 2023 REGN/JW Collaboration Programs featured today 24 Product engine generating ~1+ INDs per year Patients Enrolling; Update in 2023 *Investigational New Drug application – IND; Investigator Initiated Trial – IIT; Newly Diagnosed Multiple Myeloma – NDMM Collaboration program TSVT-owned program IND potential EOY 2023 IIT potential EOY 2023 (JW / China)

What you will hear today Clinical progress with SC-DARIC33 in patients with AML 2seventy bio’s NextGen AML approach…. packed with innovation First regulatable CAR T cell data from the clinical trial* Key questions we will address: • Initial safety and tolerability? • Can we dose RAPA to target levels and turn the system on? • Do the SC-DARIC33 T cells activate and expand? • Do they engage and kill target cells? Signal 1: Dual targeted Signal 2: Novel high antigen sensitivity regulatable CAR architecture (RESET)* Signal 3: Inducible IL-15 cytokine support* Potency of ex vivo CBL-B gene editing in CAR T cells Progress on our Hemophilia A Collaboration with Novo Nordisk Preclinical impact of CBL-B edits in CAR T cells* Supports enthusiasm for CBL-B gene editing in bbT369 (B-NHL program) First clinical application of our megaTAL technology First direct in vivo application of the megaTAL technology Key proof of concept data and pre-clinical milestones achieved Supports additional applications of our mRNA and megaTAL technology * Presentations given at 25

AML Clinical and Preclinical Developments 26 Steve Bernstein, M.D., chief medical officer Steve Shamah, Ph.D., SVP, oncology research

Acute Myeloid Leukemia is a devastating disease in desperate need of new therapeutic approaches A normal bone marrow making WBC, RBC and platelets A leukemic bone marrow packed with blasts preventing normal blood cell formation 27

Challenges in developing T-cell therapies for AML and 2seventy’s solutions Aplasia Risk T cell Persistence Challenges in AML Description of issue 2seventy cell therapy solutions AML targets are expressed on healthy myeloid lineage & progenitor cells; aplasia related toxicities are likely to emerge if targeted robustly & constitutively Regulatable system that can be turned ON & OFF designed to reduce risks associated with long term myeloaplasia AML cell therapies have shown low response durability without consolidation with SCT Regulatable CAR reduces T cell exhaustion and designed to promote memory during OFF cycle 1 2 DARIC Platform Dimerizing Agent Regulated Immunoreceptor Complexes Next-generation Regulatable CAR Separate antigen binding and signaling subunits contain drug-dependent dimerization domains Dimerizing drug (Rapamycin) required for antigen responses 28

PLAT-08: A first-in-human Phase 1 trial of SC-DARIC33 Primary Aim: Determine the maximum tolerated and biologically effective dose Eligibility: Children and young adults with relapsed/refractory CD33+ AML 29 Link to ClinicalTrials.gov for full study information, including primary and secondary outcome measures: PLAT-08: A Study Of SC-DARIC33 CAR T Cells In Pediatric And Young Adults With Relapsed Or Refractory CD33+ AML - Full Text View - ClinicalTrials.gov

What are we looking for in the early days of this trial 30 Can we dose Rapa to maintain levels within target range for DARIC activation? Does that result in DARIC dimerization, activation and expansion? Do the DARIC cells engage antigen and mediate target cell cytotoxicity? 1 2 3

Can we dose Rapa to maintain levels within target range for DARIC activation? 1 As of March 17, 2023, three patients had received lymphodepletion (LD) and SC-DARIC33 therapy at dose level 1 (1 x 106 SC-DARIC33 T cells/kg). Rapamycin dosing was adjusted by the treating physician to attain target levels. Infusions were generally well tolerated without occurrence of dose limiting toxicities. Improvement in Rapamycin targeting 31

Compared to blood, SC-DARIC33 T cells (VHH+FRB+) were increased among T cells in tumor tissue. Patient S002Do therapeutic Rapa levels result in DARIC dimerization, activation and expansion? 2 32

Progressive hemorrhagic necrosis of the chloromas Do the DARIC cells engage antigen and mediate target cell cytotoxicity? Chloroma: Two extramedullary leukemic “tumors” in the skin above the eyelid Patient S0023 33

Percentage of total lymphocytes that are DARIC cells Percentage of CD3+ T-cells that are “turned on” by Rapa (VHH+FRB+) Patient S004Does therapeutic Rapa levels result in DARIC dimerization, activation and expansion? 2 34

Transient clearance of leukemic blasts in blood Do the DARIC cells engage antigen and mediate target cell cytotoxicity? Patient S0043 35

Summary of initial PLAT-08 correlative data First three patients / Dose Level 1 Initial safety and tolerability data consistent with CAR T cell approaches We can dose RAPA to target levels and turn the system on SC-DARIC33 T cells activate and expand SC-DARIC33 T cells traffic to, engage and kill target cells Next Steps Explore SC-DARIC33 at DL2 (5e6 cells/kg) and continue dose escalation Continue to develop next generation solutions to the additional problems that may limit efficacy 36

Our Next-Gen AML (NG-AML) program builds on SC-DARIC33 success Rapamycin-based CAR regulation 1 Solving for aplasia risk Signaling via the native T Cell Receptor 2 Solving for reduced antigen expression Dual Antigen Targeting 3 Solving for disease heterogeneity Soluble IL-15 for better T cell persistence 4 Solving for T cell persistence 37

The NG-AML CAR is tightly controlled by rapamycin dosing Rapamycin-based CAR regulation 1 38

Signaling via the native T Cell Receptor The NG-AML CAR uses the RESET architecture for higher antigen sensitivity The Rapamycin-Enabled, Switchable Endogenous TCR (RESET) architecture demonstrates enhanced sensitivity to low target density limiting low antigen escape 2 39

The NG-AML CAR recognizes CD33 and CLL-1 to address AML heterogeneity ➢ CD33 and CLL-1 have high expression and prevalence in AML blasts and Leukemic Stem Cells (LSCs) ➢ CD33 binder targets the conserved C2 domain AML LSCsAML Bulk C D 33 C L L 1 C D 33 C L L 1 Perna et al, Cancer Cell 2017 Dual Antigen Targeting 3 40

Dual Antigen Targeting The NG-AML CAR recognizes CD33 and CLL-1 to address AML heterogeneity CD33: CLL-1: 3 41 ➢ CD33 and CLL-1 have high expression and prevalence in AML blasts and Leukemic Stem Cells (LSCs) ➢ CD33 binder targets the conserved C2 domain

Soluble IL-15 for better T cell persistence The NG-AML CAR incorporates IL-15, resulting in more potent T cells IL-15 activity is regulated by a T cell activation-dependent synthetic promoter Addition of iSynPro-IL15* improves in vivo AML tumor control 4 42

27T32 27T32 Our Next-Gen CAR T for AML: Bold and packed with innovations Solving reduced antigen expression Novel architecture enhances sensitivity to low target density Regulatable system reduces T cell exhaustion and promotes memory Solving T cell exhaustion Regulatable system reduces risks from long term myelodysplasia Solving aplasia risk Inducible IL-15 Increases activation-dependent expansion & function Solving T cell persistence Solving disease heterogeneity Dual-targeting limits potential antigen escape 43

Ex Vivo and In Vivo Gene Editing Applications 44 Mike Certo, Ph.D., VP, head of genome editing

megaTAL Platform: Engineering activity and specificity 45 Gene Editing Architecture megaTAL attributes Protein Engineering for Activity and Specificity TAL Array Anchor Meganuclease Binding Meganuclease Cleavage 10-12 bp TAL target recognition 22 bp MN target recognition Creates a 4 bp, 3’ overhang DSB Small size compatable with ex and in vivo delivery modalities including AAV 2.5 kb 10-12 bp TAL target recognition Each megaTAL is a fully orthogonal reagent Single, monomeric component drives target recognition and cleavage Unbiased off-target discovery In silico off-target discovery megaTAL refinement Add Mg2+ Inactive enzyme Active enzyme

Gene Editing Ex Vivo vs In Vivo 46 Ex Vivo In Vivo 2 1 3 4 Extract cell Collect blood Grow culture Engineer cells through viral or nonviral methods 4 Re-introduce modified cells 1 Direct delivery using viral or non-viral vehicle Viral delivery Adeno Associated Virus (AAV) Non-viral delivery Lipid nanoparticle (LNP)

Gene Editing Ex Vivo vs In Vivo 47 Ex Vivo In Vivo 2 1 3 4 Extract cell Collect blood Grow culture Engineer cells through viral or nonviral methods 4 Re-introduce modified cells 1 Direct delivery using viral or non-viral vehicle Viral delivery Adeno Associated Virus (AAV) Non-viral delivery Lipid nanoparticle (LNP)

bbT369: Autologous CAR T product purpose-built to address significant need in b-NHL 48 Challenges in bNHL CAR T Description of issue CD19 Loss Target-Antigen Downregulation Poor outcomes in patients with Challenging TME and/or Aggressive disease ~30% of CD19 CAR T relapse patients have CD19 negative disease. CD19-Low tumors have been shown to escape CAR T detection and killing. PFS / OS in patients with aggressive disease characteristics, such as higher disease burden and extra-nodal sites have significantly worse outcomes CD19 CAR T cells have improved outcomes for patients with relapsed/refractory diffuse large B-cell lymphoma (DLBCL), but 60-70% of patients do not achieve a long-term durable remission, highlighting the need for additional treatment options that provide more deep, durable complete responses.1-2 1 2 3 2seventy cell therapy solution: bbT369 Novel combination of antigens to address antigen escape. Shared CD8 TM domain facilitates heterodimirization of CARS via di-sulphide bond Optimized antigen receptor signaling domains to augment T cell activation. Gene edit to enhance potency and reduce T cell exhaustion. TARGET(S) Dual target: CD20, CD79a TECH Dual targeting with split 41BB and CD28 costim Cblb gene edit for expansion, antigen sensitivity, performance INDICATION B-NHL STATUS Ph1 trial active PARTNER 2seventy owned References 1. Neelapu et. al., NEJM 2018 2. Schuster et al. NEJM 2019

Data presented at ASGCT demonstrate potential of the CBLB edit to maintain CAR T activity across multiple challenging tumor scenarios 49

CRC-403 study in B-NHL open and enrolling CRC-403: A Phase 1/2 Study of bbT369 in Relapsed and/or Refractory B-Cell Non-Hodgkin Lymphoma (B-NHL) QUESTIONS Is the safety and tolerability of bbT369 in line with prior CAR Ts? Does bbT369 show anti-B cell activity in R/R B-NHL patients? Does bbT369 show deep and durable responses? Does the dual-targeting CAR architecture limit antigen escape? Do CBLB edited T cells expand and persist? FEATURES First in human application of four 2seventy bio innovations: • Dual targeted T cell • Split-costimulation signaling architecture • MegaTAL gene editing tech • CBLB edited T cell All four are believed to have application across our research pipeline, including enhanced liquid tumor settings and solid tumors Update from Phase I CRC-403 study anticipated by the end of 2023 STUDY DESIGN Target enrollment: n=50 4 study sites Relapsed/Refractory B-cell NHL after autologous SCT or ≥ 2 prior lines of therapy B-cell NHL according to WHO 2017 classification Prior CD19 CAR-T therapy is permitted Enrollment Aphersis Lympho- depletion bbT369 DLT Period Respond assessment Day 5-2 Day 0 Day 0-30 Day 30 50 Key Questions / Features

Gene Editing Ex Vivo vs In Vivo 51 Ex Vivo In Vivo 2 1 3 4 Extract cell Collect blood Grow culture Engineer cells through viral or nonviral methods Re-introduce modified cells 1 Direct delivery using viral or non-viral vehicle Viral delivery Adeno Associated Virus (AAV) Non-viral delivery Lipid nanoparticle (LNP)

Gene Editing Ex Vivo vs In Vivo 52 Ex Vivo In Vivo 2 1 3 4 Extract cell Collect blood Grow culture Engineer cells through viral or nonviral methods 4 Re-introduce modified cells 1 Direct delivery using viral or non-viral vehicle Viral delivery Adeno Associated Virus (AAV) Non-viral delivery Lipid nanoparticle (LNP)

Hemophilia A Severe and debilitating genetic bleeding disease caused by the absence of the critical clotting molecule Factor VIII 53 Phenotypes Therapies BRUISING that can lead to hematoma SPONTANEOUS INTERNAL BLEEDING which can be life threatening if in vital organs EXCESSIVE BLEEDING following injury or surgery REPEATED BLEEDING into muscles and joints, which can lead to disability and arthropathy Bispecific antibodies can be used to replace FVIII function and prophylactically prevent bleeds, but is not suitable for traumatic and surgical bleed management FVIII replacement therapy can be given in response to an injury or prophylactically to prevent bleeding

Addresses gaps in SOC and AAV Only Hem A approaches 54 DURABLE expression without activity Troughs AAV Episomal transgene expression declines, cannot be redosed and not suitable for Pediatrics i.v. FVIII ERT requires repeat dosing and has deep troughs – acute bleeds and joint deterioration over time. Extended half-life products reduce injections but still have gaps GTx intends to have durable normalized FVIII expression i.v. FVIII i.v. EHL-FVIII F8-GE Adeno-associated virus (AAV) Therapeutic Transgene 0 50 100 150 FVIII Activity of EMA Approved Hemophilia AAV Gene Therapy Months % F V II I E x p re s s io n 6 12 24 36 n o rm al F V III le vels Mean Median Sources: EMA EPAR , ERT SoC leaves gaps in protection that GTx tries to address

Direct delivery using viral or non-viral vehicle In Vivo Gene Therapy for Hemophilia A Product concept In Vivo Gene Therapy Non-viral delivery Lipid nanoparticle (LNP) Viral delivery Adeno Associated Virus (AAV) 55 Development Responsibilities in partnership with Potentially lifelong correction of FVIII deficiency

Complimentary co-creation partnership to bring next-generation Hemophilia therapies to patients: Built around shared vision and transformational science Leveraging 2seventy’s gene therapy expertise and Novo’s deep clinical experience in hemophilia 2seventy & Novo Nordisk Collaboration Overview Research Agreement Collaboration Agreement Partnership launched with Research Agreement to “make things happen fast”! Team health, program success and scientific progress provided opportunity to enter Collaboration Agreement with defined development milestones 56

megaTAL mRNA LNP Vector Genome Capsid Delivering best-in-class liver knock-in approaches for Hemophilia A Scientific Considerations 57 LNP/megaTAL - mRNA AAV-Transgene

Exploring Product Component Designs megaTAL Enzyme mRNA LNP Formulations AAV - genome mRNA concentration % IN D E L mRNA concentration % I N D E L % Integration F V II I a n ti g e n LNP1 LNP2 T a rg e t P ro te in ( n g /m g ) Gen 2 Gen 1 Gen 2 Gen 1 Gen 2 Gen 1 Gen 2 Gen 1 58

In vivo gene editing PoC for FVIII expression*In vivo gene editing PoC for FVIII expression Mouse proof-of-concept Bleed normalization Key Characteristics of the preclinical study P B S A A V-N 8 + L N P -m eg aT AL P os iti ve c trl 0 25 50 75 100 % o f n o rm a l P B S A A V-N 8 + L N P -m eg aT AL P os iti ve c trl 0 10 20 30 40 m in u te s FVIII antigen Bleeding time Anaesthetised HA mice P B S A A V-N 8 + L N P -m eg aT AL P os iti ve c trl 0 2000 4000 6000 8000 10000 B lo o d l o s s ( n m o l, H g b ) Blood Loss Anaesthetised HA mice • AAV-N8 + LNP-megaTAL leads to integration of N8 gene in surrogate mouse alleles • Duration of effect is not addressed in this study • 1st-Gen mouse-model specific megaTAL reagent and AAV normal 59 *Positive ctrl - FVIII sequence anticipated from literature, delivered by AAV8 vector; Bleeding under anaesthesia. Data from 1 week post treatment.

“Novo Nordisk is proud to work in such close and creative collaboration with the team at 2seventy. We are thrilled this program has achieved the strong proof of concept data, triggering this important milestone event.” – Marcus Schindler, Executive vice president research and early development, and chief scientific officer, Novo Nordisk Data generated to date reach pre-established POC milestone criteria Pre-clinical proof of concept achieved across several metrics including integration, tolerability, LNP delivery technology and robust efficacy in multiple different animal models • Collaboration will continue to optimize the drug product towards pre-defined “option” criteria $15 Million Preclinical Milestone triggered in the Novo Nordisk collaboration on Hemophilia A Data show further validation of our megaTAL gene editing and in vivo mRNA platforms • Learnings and platform improvements can be leveraged for future oncology applications within 2seventy Potential for expansion of our in vivo editing platform into additional indications. Integration Metrics Efficacy Metrics Tolerability Metrics LNP Metrics 60

D ev el o p m en t C o n si d er at io n s In vivo gene editing approaches potential platform expansion Same approach used for Heme A allows leveraging existing megaTAL, LNP, and AAV improvements POC with the Hem A program provides a development path. Simpler LNP/mRNA only drug product Leverage LNP IVGE progress in HemeA, lead LNP and mRNA process remains the same Delivery to non-liver tissue opens up broader indication potential Small monomeric megaTAL is easily packaged and delivered by AAV Other emerging delivery modalities offer optionality Ex-Liver Knock Out/InLiver Knock In Liver Knock Out + 61

Summary 62 Clinical progress with SC-DARIC33 in patients with AML 2seventy bio’s NextGen AML approach…. packed with innovation First regulatable CAR T cell data from the clinical trial* Key questions addressed: • Initial safety and tolerability in line with CAR T cell approaches • We can dose RAPA to target levels and turn the system on • SC-DARIC33 T cells activate and expand • SC-DARIC33 T cells traffic to, engage and kill target cells Integration of innovations to create product 27T32 for AML: • Signal 1: Dual targeted • Signal 2: Novel high antigen sensitivity regulatable CAR architecture (RESET)* • Signal 3: Inducible IL15 cytokine support* Potency of ex vivo CBL-B gene editing in CAR T cells Progress on our Hemophilia A Collaboration with Novo Nordisk Preclinical impact of CBL-B edits in CAR T cells* Supports enthusiasm for CBL-B gene editing in bbT369 (B-NHL program) First clinical application of our megaTAL technology First direct in vivo application of the megaTAL technology Key proof of concept data and pre-clinical milestones achieved Supports additional applications of our mRNA and megaTAL technology * Presentations given at

2seventy bio’s R&D philosophy 63 Accelerate with Industry Leading Partnerships Identify the Key Problem Layer Innovations Learn Fast in the Clinic Understand tumor resistance Explore new biology Focus on the hard problem Define clear hypotheses Deploy our unique toolbox Deliver multi-nodal solutions 270-MPH (Internal DP Mfg) Turbo-charge Clin. Translation Operational flex & efficiency

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