MCL-1 is essential for efficient hematopoietic reconstitution of lethally irradiated mice. (A) Competitive reconstitution strategy. Test (wild-type, Bcl-x^+/−, or Mcl-1^+/−, all C57BL/6-Ly5.2) and competitor (wild-type, C57BL/6-Ly5.1) bone marrow cells were mixed 1-to-1 and each injected into 3 lethally irradiated C57BL/6-Ly5.1 recipient mice (R1). At 16 weeks, the bone marrow cells from the 3 competitively reconstituted recipient mice were pooled and injected into a further 3 lethally irradiated C57BL/6-Ly5.1 recipient mice (R2). Analysis for R1 and R2 was performed after 8 and 16 weeks. (B-E) Flow cytometric analysis of the contribution of test bone marrow cells to repopulate the (B) lymphoid, (C) myeloid, and (D-E) hematopoietic stem/progenitor (LSK) cell compartments in transplant recipients. (F) Lethally irradiated C57BL/6-Ly5.1 recipient mice were reconstituted with titrated numbers of either Mcl-1^+/− or wild-type (both C57BL/6-Ly5.2) bone marrow cells and the proportions of donor-derived leukocytes determined after 8 weeks by flow cytometric analysis. Staining with antibodies against B220 and Mac-1 was used to identify B cells and monocytes, respectively. (B-E) n = 15 (wild-type), 4 (Bcl-x^+/−, Mcl-1^+/−) biological samples (each circle represents the mean of 3 replicate mice). Pooled results from 7 independent experiments or (F) for each cell number: 2.5 × 10⁴, 7.4 × 10⁴, 2.2 × 10⁵, 6.7 × 10⁵, 2 × 10⁶, 6 × 10⁶; n = 2, 3, 2, 3, 6, 3, respectively (wild-type), and n = 3, 3, 4, 4, 8, 4, respectively (Mcl-1^+/−). Data represent mean ± SEM. *P < .05 Student t test with the color indicating the genotype compared with wild type. Representative FACS plots are shown.