The effect of SCID subtype and use of conditioning on T- and B-cell chimerism. Patients with SCID who undergo HCT without conditioning have engraftment of donor-derived cells in the thymus, giving rise to donor-derived T cells (purple). HSCs and B cells, if present, remain largely of recipient origin (blue). With conditioning, donor HSCs can engraft, giving rise to donor-derived B cells, and also, they can continuously replenish progenitors in the thymus, resulting in robust and durable T-cell production.