STAT5 signaling is essential for sCFU-E growth. (A) Proliferation increases in sCFU-E cells after phlebotomy. (B) Apoptosis decreases in sCFU-E cells after phlebotomy. In (A) and (B), sCFU-E cells from freshly isolated bone marrow were gated for analyses. (C) Inhibitors to STAT5 abolish sCFU-E growth. Sorted BFU-E and sCFU-E cells are cultured in 10 μM of inhibitors or vehicle control (dimethyl sulfoxide [DMSO]) and analyzed after 48 hours. Phosphate-buffered saline (PBS)-treated samples also are shown as controls. (D) Diagrams of EpoR(core) and EpoR(core+Y343). (E) Y343 in EpoR rescues STAT5 binding and sCFU-E expansion in EpoR(core+Y343) mice. Data represent the mean ± SD. BrdU, bromodeoxyuridine; Phleb., phlebotomized. ∗P < .05; ∗∗P < .01, Student’s t test or 2-way ANOVA.