Stage E1 progenitors hyperproliferate, whereas E2 proerythroblasts limit this expansion mechanism. (A) Primary stage E1, E2, and E3 BM cells were isolated, returned to culture, and analyzed for proliferation and doubling times. (B) E1 and E2 cell cycle distributions also were determined based on 7-amino-actinomycin D analyses using fresh bone preparations (and Kit, CD71, plus Ter119 costaining and gating). Stage E1 cells were distributed at ∼ 60% in S-phase. (C) During stage E1 → E2 → E3 cohort development, levels of Cdk4 and Cdk6 decrease, whereas p21 and p27 increase multifold. Values are mean ± SE for triplicate transcriptome profiling experiments.