Studies of Stat5a and Stat5b knockout (KO) mice and of loss-of-function mutations of STAT5B in humans underscore the importance of STAT5A in proliferation of the mammary ductal epithelium9 and of STAT5B for sexually dimorphic growth10, as well as both STAT5 proteins for normal immune homeostasis and function of T and NK cells4,11; in addition, gain-of-function mutations of STAT5B can be associated with malignancies in humans12. This evidence concerns the gene STAT5A and cancer.