EpoR-Cre–mediated Mcl1 deletion leads to lethal failure of blood development. (A) Photographs of representative Mcl1^F/F EpoR-Cre⁺ and Mcl1^F/wt EpoR-Cre⁺ embryos at E10.5, E12.5, E14.5, and E16.5. (B) Representative images of hematoxylin and eosin (H&E)–stained sections from E12.5 embryos showing size and gross morphology of liver (original magnification ×2) (i), areas of apoptosis of liver (black arrowheads; original magnification ×60) (ii), and atrium of heart showing number and phenotype of RBCs (original magnification ×60) (iii). (C) Cytospin preparations from livers from E10.5 (upper panels) or E13.5 (lower panels) embryos (original magnification ×40, Wright-Giemsa stain). White arrowheads denote nonnucleated mature cells, and black arrowheads denote nucleated primitive cells. (D) Blood smears of whole blood from E13.5 embryos (original magnification ×40, Wright-Giemsa stain). White arrowheads denote nonnucleated mature cells, and black arrowheads denote nucleated primitive cells. (E) Intracellular staining of hemoglobin ε chain in E10.5 (left panel) or E13.5 (right panel) fetal livers. Control bar is isotype stain; n > 5 embryos per group. Bars represent mean ± standard error of the mean (SEM). ****P < .001, 1-way analysis of variance (ANOVA) with α = 0.05. (F) Genomic PCR for floxed or deleted allele of Mcl1 from E10.5 fetal liver cells. (G-H) Methylcellulose colony-forming assays were performed using total fetal liver cells and media as indicated; each point represents a plate (n > 12). Data are mean ± SEM. (G) Enumeration of BFU-Es grown on MethoCult GF M3434 (contains EPO). ****P < .001, 1-way ANOVA with α = 0.05. (H) Enumeration of granulocyte-macrophage progenitor cells (CFU-M and CFU-GM) grown on MethoCult GF M3534 (without EPO). ****P < .001, 1-way ANOVA with α = 0.05.  (I) Enumeration of cells within the live gate by flow cytometry of fetal liver cells (n = 8-12 embryos from 4 litters). Data are mean ± SEM. ****P < .001, Student t test. ns, not significant; WT, wild-type.