Id1 overexpression in fetal liver cells. (A) Expansion of lineage-negative E14.5 fetal liver cells in vitro after retroviral transfection with an Id1 expression construct or a control (MIG) vector. (B) Relative expansion of Id1- or control vector-expressing fetal liver cells during 48-hours of culture under low (0.01 U/mL) erythropoietin conditions. Shown is the mean and SEM for 3 independent experiments, each performed in triplicate (*P < .05; see “Results” for details). (C) Representative FACS plots for ter119 and CD71 expression in lineage-negative fetal liver cells overexpressing Id1 or a control vector after 24 or 48 hours of in vitro culture. Bar charts indicating the proportion of cells at each of the 5 stages of erythroid differentiation (R1-R5) after 24 (D) and 48 hours (E) of in vitro culture of lineage-negative E14.5 fetal liver cells retrovirally transfected with Id1 expression or control constructs. Data (mean and SD) from a representative set of experiments performed in triplicate are shown.