Risk factors for survival of adenosine deaminase deficient (ADA) patients with severe combined immunodeficiency (SCID) treated with hematopoietic stem cell transplantation (HCT). MMUD indicates mismatched unrelated donor; MAC, myeloablative conditioning; IRC, intensity reduced conditioning; MSD, matched sibling donor; MFD, matched family donor; and MUD, matched unrelated donor. Double small arrows MAC indicate lower survival versus the single small arrow with IRC.