Preparative regimens. Participants in this study (n = 43) received 1 of 2 preparative regimens: (A) FLU/^TBU (n = 25), which consisted of intravenous fludarabine 40 mg/m² per day in combination with intravenous busulfan on days −5 until −2 (busulfan was targeted to steady-state concentrations of 800 to 900 ng/mL) or (B) fractionated TBI (n = 18) at a cumulative dose of 12 Gy or higher (12 Gy, n = 13; 13.2 Gy, n = 5) on days −5 until −2. In general, patients with myeloid malignancies received the FLU/^TBU regimen, whereas those with lymphoid malignancies received TBI. G-CSF–mobilized peripheral blood mononuclear cells (≥5 × 10⁶ CD34 cells/kg recipient weight) were infused on day 0. Cyclophosphamide (Cy) was given at 50 mg/kg per day intravenously on days 3 and 4 after transplant. An intravenous loading dose of CSP was given on day 5, followed by subsequent twice daily dosing adjusted to maintain whole blood trough concentrations at 120 to 360 ng/mL. In the absence of GVHD, CSP doses were tapered from day 56 through day 126.