PB was obtained from all homozygous fetuses/neonatal pups at the indicated time-points, stained with CD71/TER119 antibodies and analyzed by FACS. Each differentiation stage (R1~R5) was gated as previously described [8]; R1 (CD71mid/TER119negative)-primitive progenitor cells and proerythroblasts, R2 (CD71high/TER119negative)-proerythroblasts and early basophilic erythroblasts, R3 (CD71high/TER119positive)-early and late basophilic erythroblasts, R4 (CD71mid/TER119positive)-chromatophilic and orthochromatophilic erythroblasts, and R5 (CD71low/TER119positive)-late orthochromatophilic erythroblasts and reticulocytes. Numbers are the averages of each differentiation stage in mice derived from multiple litters. M, wtH, mtH denote mEpoR, wtHEPOR, and mtHEPOR genotype, respectively. * indicates p<0.05.