Signal Transducer and Activator of Transcription 5 (STAT5, consists of isoforms a and b) is a ~95kD member of a family of transcription factors that are activated to regulate gene transcription in response to multiple hematopoietic cytokines and growth factors. STAT5 is involved in normal cell cycle, survival and apoptosis and is implicated in leukaemogenesis and angiogenesis, as it has been found to be constitutively active in many leukemic cell lines. STAT5 is activated by phosphorylation of tyrosine 694 in STAT5a (tyrosine 699 in STAT5b), a major Src phosphorylation site that is critical for STAT5 dimerization, translocation and binding to DNA. Phosphorylation of tyrosine 694 is also important in mediating erythropoietin-induced signaling and prolactin-induced development and differentiation of mammary epithelial cells.