Base-Edited Hematopoietic Stem/Progenitor Cell X-Linked Severe Combined Immunodeficiency Gene Therapy

Stem Cell Mobilizing Agent: Subcutaneous administration for 2 consecutive days to improve stem cell collection.

Stem Cell Mobilizing Agent: Subcutaneous administration for 6 consecutive days. It is necessary to mobilize stem cells for collection.

Mucositis prophylaxis: As is standard practice prior to busulfan conditioning, IV infusion of keratinocyte growth factor (palifermin) will be administered at 60 micrograms/kg/day for 3 days before initiation of busulfan (days -6 to -4), as well as for the 3 days following study agent administration (days 1 to 3).

Transplant Conditioning Agent: An alkylating chemotherapy drug to enhance engraftment of the study agent (base-edited stem cells). For this study, busulfan is administered once daily (3 mg/kg) x 2 days, targeting a daily busulfan AUC of 4500-6500 micromol\*min/L or a cumulative AUC of 9000 micromol\*min/L (for the 2 days of therapy if levels are available. Busulfan will be infused over 3 hours each day as per standard clinical practice.

Investigational/Study Agent: Base-edited autologous CD34 plus hematopoietic stem and progenitor cell product. A one-time dose \>5(SqrRoot) 10\^6 base-edited cells/kg body weight will be administered to each participant. The exact dosage depends on the number of viable cells that are repaired, cryopreserved, and thawed. The study agent will be administered by IV infusion in a volume of approximately 50 mL over about 15-30 minutes, in accordance with NIH CC DTM infusion policy.