EPO dose-dependent delays in EPOR-HM erythroblast cell-cycle progression. (A) Defective EPO-dependent cell-cycle progression among EPOR-HM erythroblasts. Erythroid progenitor cells from wt-EPOR, EPOR-HM, and EPOR-H marrow preparations were expanded in SP34-EX. At day 2, erythroblasts were 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 hours, cell-cycle distributions for CD71^highTer119^neg erythroblasts were determined. Values are means (± SD) for n = 3 independent cultures (from n = 3 mice for EPOR alleles). In lower subpanels, cell-cycle distributions are summarized for wt-EPOR versus EPOR-HM versus EPOR-H. (B) Representative primary data for cycle distributions also are shown for wt-EPOR, EPOR-HM, and EPOR-H erythroblasts.