Tyrosine mutants of the EPOR and effects in cell culture. (A) Schematic of the parental cEPOR containing an intact cytoplasmic tail and the R129C substitution in the extracellular domain (WT), the cEPORYF mutant in which all eight cytoplasmic tyrosines have been replaced by phenylalanines (YF), and various mutants retaining selected tyrosine residues as shown (Y1F, YF:1Y, YF:1-4Y, YF:5-8Y, and YF:8Y). For convenience, specific tyrosine positions in the cytoplasmic segment have been numbered 1-8 as indicated. (B) Immunoblot analysis showing comparable levels of the cEPOR variants in transfected BaF3 cell lines. Arrow, EPOR. (C) Titers of SFFV-cEPOR viruses containing the tyrosine variants described above, as determined in infection analyses with fibroblasts. The schematic represents a genetic map of these recombinant retroviruses. The immunoblot displays detection of the cEPOR proteins from the indicated viruses.