ESR1 and breast cancer: As mammary stem and/or progenitor cells are thought to be the targets of malignant transformation in the breast [119] and overexpression of ERα is thought to be the first event in the development of ER positive breast cancers, the ability of in utero exposure to cadmium to cause an expansion in the mammary stem/progenitor cell population and an increase in the expression of ERα suggests that in utero exposure to the metal may predispose the mammary gland to malignant transformation.