Erythroid colony formation from CFU-E by chimeric receptors. (A) Fetal liver cells were infected with retrovirus encoding the EGFR-EPOR chimeric receptor and stained with anti-EGFR antibody (Amersham, Uppsala, Sweden). The EGFR-positive cells were sorted by FACS. P shows EGFR-positive cell populations used for the CFU-E assays. (B) The EGFR-positive cells shown as P in (A) were reanalyzed. The vertical axis is cell numbers and the horizontal axis is FITC fluorescence. (C) Sorted 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 by dividing the CFU-E colony numbers obtained with the EGF stimulation by that of EPO stimulation. The means ± SEM for three experiments are shown.