Inclusion Criteria:

• 1. Age 18 to 35 years at the time of consent
• 2. Diagnosis of transfusion dependent beta thalassemia (β0 β0, β+β0, β+β+, βEβ0, βEβ+). Transfusion-dependent is defined as a history of receiving at least 120 mL/kg/year packed red blood cells or at least 8 transfusions per year in the past two years. The first 2 subjects enrolled must have a non- β0 β0 genotype.
• 3. Genetic confirmation of α and β thalassemia diagnosis (β0β0, β+β0, β+β+, βEβ0, Eβ+ dominant β-thalassemia) by a Clinical Laboratory Improvement Amendments (CLIA) certified laboratory is required.
• 4. Clinically stable, Karnofsky score 70, and eligible to undergo Hematopoietic Stem Cell Transplantation (HSCT).
• 5. Female subjects of childbearing potential must agree to use acceptable method(s) of contraception from consent through at least 6 months after CHOP-ALS20 infusion
• 6. Male subjects of reproductive capacity must agree to use effective contraception from start of mobilization through at least 6 months after CHOP-ALS20 infusion

Exclusion Criteria:

• 1. Prior receipt of HSCT or gene therapy
• 2. An available Human Leukocyte Antigen (HLA)-matched family donor
• 3. More than one alpha globin gene deletions/mutations.
• 4. Any prior or current malignancy (excluding adequately treated basal or squamous cell carcinoma of the skin)
• 5. Known cancer predisposition syndrome
• 6. Positive for HIV-1, HIV-2, Human T Cell Lymphotropic Virus-1,2 (HTLV-1, HTLV-2) or active hepatitis B or active hepatitis C infection
• 7. Clinically significant active bacterial, viral (including COVID-19 and influenza), fungal, or parasitic infection (temporary exclusion)
• 8. Clinically significant bleeding disorder
• 9. Evidence of cardiac dysfunction (left ventricular ejection fraction \<50% or shortening fraction \<27%) or clinically significant arrhythmia
• 10. Evidence of advanced liver disease (ALT \>5x the upper limit of normal (ULN), prothrombin time \>1.5 x ULN, direct bilirubin \> 3x ULN) not attributable to iron chelation therapy, or evidence of bridging fibrosis on liver biopsy or fibrosis stage of F3 or higher by magnetic resonance elastography (MRE) if obtained as part of clinical care
• 11. Liver R2 or R2 MRI or liver biopsy with liver iron concentration 15 mg/g dw (temporary exclusion)
• 12. Diffusion capacity of carbon monoxide (DLco) \<50% of predicted (corrected for Hb)
• 13. Pulse oximetry in room air \<92%
• 14. Evidence of renal dysfunction (creatinine \>1.5x ULN or Glomerular Filtration Rate (GFR) \<70 ml/min/1.73 m2 based on cystatin C/creatinine equation)
• 15. Cardiac T2 MRI \< 10 ms
• 16. Platelet count \<100,000/mcL or absolute neutrophil count \<1000/mcL except if attributed to benign ethnic neutropenia
• 17. Unable to receive red cell transfusion (significant allo/auto immunization)
• 18. Uncontrolled systemic hypertension
• 19. Uncontrolled seizure disorder
• 20. Diagnosis of a significant psychiatric disorder that could seriously impede the ability to participate in the study
• 21. Immediate family member with a known or suspected Familial Cancer Syndrome
• 22. Contraindication to anesthesia
• 23. For female subjects, pregnancy or breastfeeding
• 24. Participation in another clinical trial of an investigational drug within 30 days or 5 drug half-lives, whichever is longer, of screening (temporary exclusion)
• 25. Any other condition that would render the subject ineligible for mobilization/apheresis and/or HSCT as determined by the investigator