mPHOSPHO1 is essential for fetal erythropoiesis. (A) Smaller percentages of CD71⁺Ter119⁺ erythroblasts in E14.5 KO mouse livers. Mature red cells in fetal livers were lysed and cells were stained with anti-CD71 and anti-Ter119 antibody (n = 9 fetal livers from 3 mice per group, mean + SEM). (B) Photographs of E14.5 WT and KO embryos. (C) Complete blood counts of 0.5-day WT and KO neonates (n = 7 per group, mean + SEM). (D) Lower growth rate in erythroblasts depleted of mPHOSPHO1. E14.5 fetal liver lineage-negative cells were isolated from WT and PHOSPHO1-KO mice, expanded in maintenance medium for 1 day, and cultured in differentiation medium for 2 days (n = 3 per group, mean ± SEM). Cells were counted during the 2 days of differentiation. (E) Gene expression of WT and KO erythroblasts after 1 day differentiation (n = 3 per group, mean + SEM). mRNA expression is plotted relative to expression of 18s rRNA. (F) Enucleation is lower in KO erythroblasts after 2 days of differentiation (n = 3 per group, mean + SEM). HCT, hematocrit; NS, not significant.