STAT5A and STAT5B heterodimerization. Ba/F3-mMPL cellular extracts were generated before (−) and after (+) TPO stimulation for 15 minutes. Immunoprecipitation was performed with STAT5A- and STAT5B-specific antisera and analyzed by Western blot to detect changes in phosphotyrosine (P-TYR) content (first 4 lanes). Equivalent protein in the −/+ lanes is shown by probing a parallel blot with the same antibody used for immunoprecipitation (lower strip). In the pair of lanes to the right of the molecular weight markers, STAT5A antiserum was used for immunoprecipitation and STAT5B antiserum was used for detection.