Endocrine therapy forms the backbone of treatment of ER+ breast cancer, and includes selective oestrogen receptor modulators (e.g., tamoxifen), selective oestrogen receptor degraders (e.g., fulvestrant), aromatase inhibitors (e.g., letrozole, anastrazole, exemestane) and ovarian function suppression (e.g., goserelin). Here, CYP19A1 is linked to breast cancer.