Analysis of wild-type and Rb^–/– erythroid cocultures.Rb^–/– or wild-type erythroid progenitors were CFSE labeled and mixed with combinations of Rb^–/– or wild-type unlabeled cells (combinations 1 to 4). (A) FACS scheme used to discriminate nucleated erythroid cells, enucleated erythrocytes, and extruded “free” nuclei based on the DNA dye DRAQ5 and forward scatter on day 2 of differentiation. Note the near absence of enucleated and free nuclei in differentiated Rb^–/– erythroid cells. CFSE-labeled (+) and unlabeled (–) cells were separated based on fluorescence intensity. (B) Cell cycle profiles for each population measured by DRAQ5 analysis of nucleated erythroid cells. (C) Enucleated cell percentage based on gate indicated in panel A. Error bars indicate SD. These data are representative of 2 independent experiments.