Approximate developmental stages recapitulated during G1E-ER4 cell maturation. The histograms represent the average signal level of selected erythroid transcripts at indicated time points, as predicted by the current microarray study. The shaded envelopes drawn around the histograms are based on previously reported expression kinetics determined in other models of erythropoiesis.^52-54  The expression pattern observed during G1E-ER4 cell maturation is consistent with the transition from late BFU-E to basophilic erythroblast stages. Pro indicates proerythroblast; Baso, basophilic erythroblast; Poly, polychromatophilic erythroblast; Ortho, orthochromic erythroblast; and Retic, reticulocyte.