Expression of oncogenic K-ras from its endogenous promoter leads to a partial block of erythroid differentiation and hyperactivation of Epo-dependent signaling pathways. At E15.5, fetal liver cells were isolated from individual embryos that had been induced with doxycycline for the previous 42 to 48 hours. Cells were simultaneously stained for CD71 and TER119, and erythroid differentiation was analyzed (A). The percentages of R1 + R2 cells (predominantly primitive progenitor cells, including mature BFU-Es and CFU-Es) are shown in brackets on individual graphs (A) and further quantified (error bars represent standard deviations) (B). (C) TER119-negative cells were purified from individual E15.5 embryos, deprived of serum and growth factors for 30 minutes, then stimulated with various concentrations of Epo for 10 minutes. Phosphorylated and total levels of Stat5, p44/42 ERK, and Akt proteins were measured by Western blotting (see “Materials and methods”).