STAT5 (signal transducers and activators of transcription 5) is a member of the STAT family and this protein is phosphorylated by receptor associated protein kinases in response to cytokines and growth factors. The differentiation of T helper (Th) cells is regulated by members of the STAT family of signaling molecules. In BCR-ABL-positive cells, STAT5 is constitutively activated by tyrosine phosphorylation. STAT5 activation results in upregulation of Bcl-XL and increased resistance to induction of apoptosis. STAT5 is involved in expression and growth hormone-mediated sexually dimorphic regulation of cytochrome P450 3A10/lithocholic acid 6 beta-hydroxylase.