Expression of active and dominant negative forms of Ras inhibits erythroid colony formation from CFU-E in fetal liver cells. (A) Fetal liver cells were infected with retrovirus encoding RasV12 or RasN17 or the virus vector, and GFP-positive cells were collected by FACS. P shows GFP-positive cell populations used for the CFU-E assays. (B) The GFP-positive cells were subjected to in vitro colony assays. The relative CFU-E colony numbers were calculated as in Fig 2C.