Stat5a and Stat5b (signal transducer and activator of transcription 5a and 5b; 96% homology) undergo receptor tyrosine kinase or G protein-coupled receptor-dependent phosphorylation in response to cytokines or growth factors, and then form homo- or heterodimers that translocate to the nucleus where they initiate transcription. Activation of Stat5a via IL-2, IL-3, IL-7, GM-CSF, erythropoietin, thrombopoietin and growth hormones influences proliferation, differentiation and apoptosis in lymphohematopoietic cells. Activation of Stat5b via IL-2, IL-4, CSF1 and growth hormones influences TCR signaling, apoptosis, adult mammary gland development and sexual dimorphism of liver gene expression. Stat5b is the major liver-expressed Stat5 form that has been shown to fuse with retinoic acid receptor-alpha gene in acute promyelocytic leukemias (APLL).