The evidence for a contribution of prolactin and STAT5 to breast cancer cell survival, breast cancer progression and to chemotherapeutic response is strengthened by our observations that prolactin treatment of human breast cancer cells regulates a number of genes associated with cancer progression, including the therapeutically important target gene, HSP90A. HSP90α is important for malignant progression in breast cancer, but when elevated in untransformed mammary epithelial cells may participate in a switch between survival and apoptosis. This evidence concerns the gene STAT5B and breast cancer.