Expression of constitutively active MEK1 inhibits erythroid differentiation. Freshly isolated fetal liver TER119^- cells were infected with bicistronic retroviruses encoding hCD4 alone, H-ras.V12, ca.MEK, ca.Akt, or ca.Rlf and cultured for 2 days on fibronectin-coated plates; Epo was added during the first day.⁵  The cells were then simultaneously stained with a fluorescein isothiocyanate (FITC)-conjugated anti-CD71 monoclonal antibody (mAb), a phycoerythrin (PE)-conjugated anti-TER119 mAb, and an anti-hCD4 mAb followed by cyanine 5 (Cy5)-conjugated donkey anti-mouse immunoglobulin G (IgG) antibody. Their differentiation profiles were analyzed by flow cytometry. Dead cells (7-aminoactinomycin D [7-AAD]-positive), debris, and most terminally differentiated reticulocytes (low forward scatter) were excluded from analysis. The panels display the density plots of infected cells (hCD4⁺ cells). The percentages of TER119^- cells (presented as total of R1 and R2 cells) and TER119⁺ cells (presented as total of R3-R5 cells) are labeled at the bottom of each density plot.