JAK2 plays an essential role in EPO-mediated erythroid differentiation. (A) Fetal liver cells infected with retrovirus encoding ▵JAK2 or JAB 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) Sorted GFP-positive cells were subjected to in vitro colony assays. The relative CFU-E colony numbers were calculated as in Fig 2C. (C) Fetal liver cells infected with retrovirus encoding EGFR-JAK2 were stained with anti-EGFR antibody, and the EGFR-positive cells were sorted by FACS. P shows EGFR-positive cell populations used for the CFU-E assays. (D) The EGFR-positive cells were subjected to in vitro colony assays without cytokine or with EPO or EGF at a concentration indicated. The relative CFU-E colony numbers were calculated as in Fig 3B.