IRP1 deficiency stimulates erythroid cell differentiation in the spleen. WT, IRP1^−/−, and IRP2^−/− mice (n = 7) were euthanized at the age of 4 to 6 weeks, and splenic cells were analyzed by fluorescence-activated cell sorter. (A) CD71 and TER119 staining. (B) Region I: the proerythroblast population (CD71^high TER119^low). (C) Region II: the basophilic erythroblast population (CD71^high TER119^high). (D) Region III: the hemoglobin producing polychromatic and orthochromatic erythroblasts (CD71^low-medium TER119^high). (E) (Left) Western blots of IRP1, TfR1, and β-actin in splenic tissue from WT, IRP1^−/−, or IRP2^−/− mice (n = 4). (Right) Densitometric quantification of TfR1 expression. **P < .01; ***P < .001.