JAK2 and breast cancer: While it explains the dichotomous effects of early versus late-age pregnancy on breast cancer risk, the dual-role of PRL-JAK2-STAT5 in both promoting normal cell differentiation and suppressing the anticancer barrier in precancerous cells also predicts that hyperprolactinemia-inducing antipsychotics may have a similar dichotomous impact on breast tumorigenesis – reducing breast cancer risk when taken at a young age but increasing breast cancer risk if started at an older age or when early lesions have already been diagnosed.