Base Editing Hematopoietic Stem Cell and T Cell Gene Therapy for CD40L-HyperIgM Syndrome: Single Patient Study

The study cell product is base edited autologous HSPCs which will be administered as a one-time infusion following myeloid conditioning.

Serotherapy agent, 10 mg/m\^2 on days -21, -20 and -19

Immunophilin drug, will start on day -1, targeting a trough level between 4-12 ng/mL.

Mucositis prophylaxis agent, will be administered at 60 mcg/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).

Myeloid conditioning agent, administered once daily (3 mg/kg) x 2 days, targeting a daily AUC of 4500 micromol\*min/L or a cumulative AUC of 9000 micromol\*min/L for the 2 days of therapy, if levels are available

The secondary study cell product is base edited autologous which will be administered as a one-time infusion two weeks following the infusion of the base-edited autologous HSPCs.