In mammals, STAT5 proteins have been associated with many functions including cell differentiation, lipid mobilization, and lymphocyte development. STAT5 refers to two proteins: STAT5A and STAT5B which are both activated by phosphorylation at Tyr694 and Tyr699, respectively, by JAK2, which is activated by the binding of numerous cytokines and hormones including growth hormone (GH), erythropoietin (EPO), prolactin (PRL), and several interleukins (ILs) to their receptors. JAK2 and STAT5 are important for IL-3 and GM-CSF (granulocyte macrophage colony-stimulating factor)-regulated macrophage function. Like other STATs, STAT5 activity is regulated via inhibitory mechanisms, such as SHP-1 phosphatase dephosphorylation and negative feedback loops involving CIS (cytokine-inducible SH2-containing protein) and SOCS (suppressor of cytokine signaling) proteins. Covalent modification by acetylation and deacetylation can also modulate STAT5 activity.[1]