Signal transducer and activator of transcription (STAT), named after their dual role, generally mediate cytokine, growth factor and hormone receptor signal transduction. In mammals, seven STAT proteins have been identified. STAT5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Two highly homologous STAT5 isoforms, 96kDa STAT5a and 94kDa STAT5b, are encoded by two tandemly linked genes. Although both STAT5 isoforms are roughly 95% homologous at the level of cDNA, they exhibit both redundant and non-redundant functions in vivo, probably due to differences in their transactivation domain. Aberrant regulation of STAT5 has been observed in solid tumors as well as in patients with either chronic or acute myeloid leukemia. Kinase inhibitors are currently being developed to negatively regulate STAT5 activity for clinical purposes.