This randomised, multicentre, double-blind, placebo-controlled, phase 2 trial recruited postmenopausal adult women aged at least 18 years with ER-positive, HER2-negative, metastatic or locally advanced inoperable breast cancer and an Eastern Cooperative Oncology Group performance status of 0–2, who had relapsed or progressed on an aromatase inhibitor, from across 19 hospitals in the UK. The gene discussed is CYP19A1; the disease is breast cancer.