Stat5 is activated in response to a wide variety of ligands, including IL-2, GM-CSF, growth hormone, and prolactin. Phosphorylation at Tyr694 is obligatory for Stat5 activation. This phosphorylation is mediated by Src upon erythropoietin stimulation. Stat5 is constitutively active in some leukemic cell types. Phosphorylated Stat5 is found in some endothelial cells treated with IL-3, which suggests its involvement in angiogenesis and cell motility. Stat5a and Stat5b are independently regulated and activated in various cell types. For instance, interferon treatment predominantly activates Stat5a in U-937 cells and Stat5b in HeLa cells.