Evaluation of Efficacy and Safety of a Single Dose of CTX001 in Participants With Transfusion-Dependent β-Thalassemia and Severe Sickle Cell Disease

Key Inclusion Criteria:

* Participants with TDT and SCD:
* Eligible for autologous stem cell transplant as per investigator's judgment.
* Participants with TDT:
* Diagnosis of TDT as defined by:
* Documented homozygous β-thalassemia or compound heterozygous β-thalassemia including β-thalassemia/hemoglobin E (HbE). Participants can be enrolled based on historical data, but a confirmation of the genotype using the study central laboratory will be required before busulfan conditioning
* History of at least 100 milliliter (mL)/kilograms (kg)/year or 10 units/year of packed red blood cells (RBC) transfusions in the prior 2 years before signing the consent or the last rescreening for patients going through re-screening
* Participants with SCD:
* Diagnosis of severe SCD as defined by:
* Documented SCD genotypes
* History of at least two severe VOCs events per year for the previous two years prior to enrollment

Key Exclusion Criteria:

* Participants with TDT and SCD:
* A willing and healthy 10/10 human leukocyte antigen (HLA)-matched related donor is available per investigator's judgement
* Prior hematopoietic stem cell transplant (HSCT)
* Clinically significant and active bacterial, viral, fungal, or parasitic infection as determined by the investigator
* Participants with TDT:
* Participants with associated α-thalassemia and \>1 alpha deletion, or alpha multiplications
* Participants with sickle cell β-thalassemia variant
* Participants with SCD:
* History of untreated moyamoya syndrome or presence of moyamoya syndrome at screening

Other protocol defined Inclusion/Exclusion criteria may apply.