Selected conditioning regimens of different dose intensities. BU indicates busulfan; CY, cyclophosphamide; TBI, total body irradiation; Flu, fludarabine (various dosing schedules); AraC, cytosine arabinoside; ATG, antithymocyte globulin (or thymoglobulin); and ¹³¹I, anti-CD45 antibody conjugated to ¹³¹I. *“High-dose” TBI (800-1320 cGy). †“Low-dose” TBI (200-400 cGy).