At a late Ter119^pos stage, EPOR-HM erythroblasts falter in an apparent EPO dose-dependent G0/G1-phase accumulation. (A) Erythroblasts from marrow preparations were expanded in SP34-EX for 2.5 days, washed, and returned to culture in the absence of SCF and presence of EPO at 0.2 U/mL or 1.0 U/mL. At 24 and 48 hours, erythroblasts were analyzed for Ter119 expression and forward-angle light scatter (data shown are for wt-EPOR erythroblasts). Based on FALS, 3 subpopulations were resolved and are designated A, B, and C. (B,C) For preparations from wt-EPOR, EPOR-HM, and EPOR-H marrow, cell-cycle distributions were determined for Ter119^pos stage-A erythroblasts (panel B) and Ter119^pos stage-B erythroblasts (panel C). Values are means (± SD) for n = 3 independent cultures (from n = 3 mice for EPOR alleles).