Impaired proliferation of differentiating E2f4^–/– fetal erythroid cells in vitro. FL erythroid cells pooled from E2f4^+/+ and E2f4^–/– E12.5 embryos were expanded and induced to differentiate in vitro. Proliferation, viability, and cell cycle parameters were measured at 0, 12, 24, 48, and 72 hours following differentiation induction. (A) Cumulative proliferation was assessed by measurement of live cell numbers in each culture by trypan blue exclusion assay. (B) Level of viability was determined by trypan blue exclusion. (C) Cell cycle profiles were measured by PI analysis, and the percentage of cells in each cell cycle stage was determined by Modfit. (D) FACS determination of BrdU incorporation by Ter119⁺ E15.5 FL cells following 1 hour in vitro BrdU pulse at 24 hours after differentiation induction. For all graphs, E2f4^+/+, closed symbols; E2f4^–/–, open symbols. Error bars indicate standard deviation.