CYP19A1 and breast cancer: Oestrogen receptors (ERs), HER-2, EGFR, VEGFR, PI3K, mTOR, 17β-HSD1, aromatase (CYP19A1) and steroid sulfatase (STS) have all been targets for the development of dual acting inhibitors.1–3 Each of the dual inhibitor target combinations was inspired by the crucial role played by the target in the viability of tumours in different subclasses of breast cancer.