Here, we designed a target trial that aims to address the same research question as one of the analyses conducted in the Breast International Group 1-98 (BIG 1-98) trial: what is the comparative risk of death between aromatase inhibitors and tamoxifen in postmenopausal women with estrogen or progesterone receptor–positive breast cancer?8. The gene discussed is CYP19A1; the disease is breast carcinoma.