Loss of PUMA confers resistance to bone marrow failure elicited by acute loss of both alleles of Mcl-1. (A) Lethally irradiated C57BL/6-Ly5.1 mice were reconstituted with the hematopoietic system of the following genotypes: Mcl-1^fl/fl;Puma^−/−, Mcl-1^fl/fl;Puma^+/−, Mcl-1^fl/fl;Bim^−/−, Mcl-1^fl/fl, Puma^−/− (all RosaCreER^Ki/+), and wild-type (CreER only). Survival of these reconstituted animals was monitored after treatment with tamoxifen to activate the latent CreER recombinase to cause deletion of the floxed Mcl-1 alleles in the hematopoietic cells. (B-C) Lethally irradiated C57BL/6-Ly5.1 recipient mice were reconstituted with Mcl-1^fl/fl;Puma^−/−, Mcl-1^fl/fl;Puma^+/−, Mcl-1^fl/fl;Bim^−/−, Mcl-1^fl/fl, Puma^−/− (all RosaCreER^Ki/+), and wild-type (CreER only) test bone marrow cells mixed 1-to-1 with competitor wild-type (GFP⁺) bone marrow cells. (B) After 8 weeks, a baseline measurement of test vs competitor bone marrow–derived contribution to leukocytes in the blood was conducted by flow cytometric analysis; tamoxifen was administered to induce Mcl-1^fl deletion. The contributions of the test- vs competitor-derived leukocytes in the blood were determined by flow cytometric analysis after 30 days. (C) After 30 days, the contributions of the test- vs competitor-derived LSK population was determined in the bone marrow of reconstituted mice by flow cytometric analysis. (A) Mice numbers (n) indicated in brackets. (B-C) Mice numbers are as follows (pre-, post-TAM): Mcl-1^fl/fl;Puma^−/− (14,14), Mcl-1^fl/fl;Puma^+/− (12,9), Mcl-1^fl/fl;Bim^−/− (6,6), Mcl-1^fl/fl (12,9), Puma^−/− (18,18), CreER only (6,3). Data represent mean ± SEM. **, ***, **** denote significant differences where P < .01, < .001, < .0001 between the indicated groups (Student t test).