EPO-dependent tyrosine phosphorylation of STAT1 and STAT5a/b. Wild-type and STAT1^–/– splenic proerythroblasts were depleted of cytokine for 4 hours and then stimulated with 2 U/mL EPO for various times. Following cell lysis, lysates were resolved via SDS-PAGE and transferred to a PVDF membrane. The membrane was probed with an antiphospho-STAT1 antibody (A). Alternatively, an immunoprecipitation was performed with peptide-specific antibodies against STAT5a (B) or STAT5b (C). Immune complexes were resolved by SDS-PAGE and transferred to a PVDF membrane. The membrane was probed with an antiphospho-STAT5 antibody. The membranes was stripped and reprobed with a peptide-specific antibody recognizing total STAT1 (A) or STAT5 (B-C).