In contrast to the effects of hormone replacement on the incidence of breast cancer, the use of either an antioestrogen, tamoxifen, to block the action of oestrogen in breast cancer (EBCTCG, 1998), or an aromatase inhibitor, to prevent oestrogen synthesis in postmenopausal patients (ATAC Trialist Group, 2002), is effective, and is considered to be the standard treatment strategy for breast cancer. Here, CYP19A1 is linked to breast cancer.