Breast tumor kinase mediates tyrosine phosphorylation of signal transducers and activators of transcription. Breast tumor kinase (Brk) mediates tyrosine phosphorylation of signal transducer and activator of transcription (STAT)3, STAT5a, and STAT5b. STAT5a/b knockout mouse embryo fibroblast (MEF5-/-) cells were transfected with (a) STAT3, (b) STAT5a, or (c) STAT5b along with either pRc (vector), Brk, K219M Brk (kinase inactive), or Y477F Brk (constitutively active). Immunoprecipitations (IPs) for STAT3, STAT5a, or STAT5b were performed using antibodies specifically directed against each STAT and were analyzed by immunoblotting with antibodies directed toward STAT3, STAT5a, STAT5b (middle), or phosphotyrosine (top). Total lysates were analyzed for the expression of the Brk construct by immunoblotting with anti-Brk antibody (bottom).