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 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