Diagram of R129C isoforms containing altered Stat binding regions. R129C is shown at left, with the R129C substitution and tyrosine residues (343 and 401) required for Stat5 activation indicated by black boxes. Other tyrosine residues in the receptor cytoplasmic region are not shown. R129C/343-S3 has the sequence GYMPQ (Stat3 binding site) substituted for residues 342–346, R129C/401-S3 has GYMPQ substituted for residues 400–404, and R129C/343/401-S3 has both substitutions (as indicated by hatched boxes). The substitutions preserve Y343 and Y401 but change residues at the −1, +1, +2, and +3 positions. All other residues are identical to wild-type (WT) EpoR.