Inclusion Criteria: * ≥12 to ≤ 40 years * Severe disease, as defined by having experienced at least one of the following SCD-related events despite appropriate supportive care measures: * recurrent severe VOC (≥ 4 episodes in the preceding 2 years) * ACS (≥ 2 episodes in the prior 2 years with at least one episode in the past year) * Lansky/Karnofsky performance status of ≥ 80 Exclusion Criteria: * Available 10/10 HLA-matched sibling donor * Prior HSCT or gene therapy * Prior or current malignancy or myeloproliferative or a significant coagulation or immunodeficiency disorder * Clinically significant and active bacterial, viral, fungal or parasitic infection * Pregnancy or breastfeeding in a postpartum female * Presence of a chromosomal abnormality/mutation that may put the participant at an increased risk for MDS or AML per investigator's judgment