STAT5B and breast cancer: STAT5 phosphorylation at tyrosine 694 (Y694) is essential for cell survival, proliferation, angiogenesis and metastasis in certain cancers of both hematopoietic and non-hematopoietic origin.1, 2 STAT5 phosphorylation can be prognostic in patients with breast cancer,3 and its overexpression promotes breast cancer formation in mice.4 These findings underline the importance of characterizing the downstream targets along the STAT5 signaling pathway and the necessity of identifying regulators of STAT5 phosphorylation.