U0126 restores erythroid differentiation in H-ras.V12-transduced erythroid progenitors. Freshly isolated fetal liver TER119^- cells were infected with a bicistronic retrovirus encoding H-ra.V12 and hCD4 and cultured as described in Figure 6. (A) After 2 days in culture in the absence or presence of U0126, erythroid differentiation profiles were analyzed by FACS as described in Figure 3. hCD4⁺ cells express H-ras.V12, whereas hCD4^- cells are uninfected and provide an internal control to the experiment. The percentages of TER119^- cells (presented as total of R1 and R2 cells) and TER119⁺ cells (presented as total of R3-R5 cells) are indicated at the bottom of each density plot. (B) After 3 days in culture in the absence or presence of U0126, the cells were processed with Benzidine-Giemsa stain. Cells from one representative field are shown here. The arrow indicates a late orthochromatophilic erythroblast, and the arrowhead indicates an enucleated reticulocyte. Scale bar: 20 μm.