In this randomised, double-blind, placebo-controlled, phase 2 trial, postmenopausal women aged at least 18 years with an Eastern Cooperative Oncology Group performance status of 0–2 and oestrogen receptor-positive, HER2-negative, metastatic or locally advanced inoperable breast cancer who had relapsed or progressed on an aromatase inhibitor were recruited from 19 hospitals in the UK. This evidence concerns the gene CYP19A1 and breast carcinoma.