ADA and severe combined immunodeficiency: • No conditioning versus conditioning regimen, and SCID genotype/phenotype: HSCT engraftment without any pretransplant conditioning regimen is theoretically possible in most SCID infants, especially if NK- SCID (e.g., ADA-SCID, common γc defect, JAK3 defect), but with frequently defective donor B cell reconstitution and so persistent B cell deficiency that requires long-term (for the rest of life) immunoglobulin replacement therapy.