We previously showed that levels of the Jak2 phosphatase, PTP1B, were inversely correlated with Nuc-pYStat5a/b levels in human breast cancer and that PTP1B suppressed prolactin-induced Stat5a phosphorylation levels [43], suggesting that low levels of Nuc-pYStat5a/b in breast cancer may in part be due to reduced phosphorylation of Stat5a. This evidence concerns the gene STAT5A and breast cancer.