In vitro differentiation of Rb^–/– erythroid progenitors. (A) Cumulative proliferation was assessed by measuring the number of live cells in each culture using trypan blue exclusion assay. (B) Level of viability 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 LT Software (Verity Software House, Topsham, ME). (D) FACS determination of BrdU incorporation after 1-hour in vitro BrdU pulse at 2 days after differentiation induction. (E) Morphologic analysis of cytospins after erythroid differentiation by May-Grünwald-Giemsa and benzidine (dark brown) staining. Arrowheads denote enucleated cells. Error bars indicate SD. These data are representative of 4 independent experiments.