Constitutively active mutant signaling proteins activate specific signaling pathways downstream of H-ras in erythroid progenitors. Freshly isolated fetal liver TER119^- cells were infected with bicistronic retroviruses encoding hCD4 alone (control vector), H-ras.V12, constitutively active (ca.) Raf, ca.MEK, ca.Akt, or ca.Rlf. The cells were cultured and harvested, and lysates were analyzed exactly as described in the legend to Figure 1. The extent of activation of each individual pathway by each ca. mutant was compared with that induced H-ras.V12. ca.Akt was created by the deletion of its PH domain and the addition of an src myristoylation signal at its N-terminus.²⁵  Therefore, the size of ca.Akt protein is smaller than that of wild-type Akt.