STAT 5b is a 90 kD member of the STAT (signal transducer and activator of transcription) protein family that is phosphorylated in response to cytokine receptor-associated kinase activity. Phosphorylation of STAT 5b induces nuclear translocation to activate transcription. STAT 5b is activated by IL-2, IL-4, CSF-1, and other growth hormones and is involved in diverse biological processes including TCR signaling, apoptosis, sexual dimorphism of liver gene expression and adult mammary gland development. STAT 5b is fused with retinoic acid receptor-alpha in some acute promyelocytic leukemias. STAT 5b is highly expressed in bulk germ cell seminoma and normal endometrium (late proliferative phase) and weakly expressed in fetal liver, testis, brain, human placenta, and muscle. STAT 5b has been shown to interact with the proteins nr3c1 and nmi. The Poly6101 antibody recognizes the N-terminal region of human STAT5b and has been shown to be useful for Western blotting.