MCL-1 is essential for recovery of the hematopoietic system from 5-FU challenge. (A) Experimental strategy. (B) Mcl-1^+/−, Bcl-x^+/−, and wild-type (control) mice were treated with 1 dose of 5-FU and survival was monitored over a 21-day period (median survival, Mcl-1^+/− mice: 10 days; log-rank [Mantle-Cox] test, P < .0001). (C) RBC numbers in Mcl-1^+/−, Bcl-x^+/−, and wild-type mice at 10 days or when sick (Mcl-1^+/−) after treatment with 5-FU. (D) Histologic analysis of the bone marrow of the mice from panels A and B at the same time point (representative images; ×40 objective, 250 µm scale bar). (E) RBC levels in the blood of 5-FU–treated Mcl-1^+/−, Bcl-x^+/−, and wild-type mice on the days indicated. n = 15 (wild type), 5 (Bcl-x^+/−), 10 (Mcl-1^+/−) mice; pooled results from 9 independent experiments. Data represent mean ± SEM. * and ** denote P < .05 or < .01, respectively, for the indicated comparisons (Student t test).