Expression of constitutively active MEK1 induces Epo-independent growth of erythroid progenitors. Freshly isolated fetal liver TER119^- cells were infected with bicistronic retroviruses encoding GFP alone, H-ras.V12, ca.MEK, ca.Akt, or ca.Rlf and cultured for 1 day. GFP⁺ cells were isolated by FACS sorting and continuously cultured in fibronectin-coated wells in the absence of Epo. Cell numbers are presented as total viable cells. Data are averages from triplicate cultures of a representative experiment and standard deviations were always less than 5% of the averages.