3). For ER-positive (ER+) patients, allocation to about 5 years of adjuvant tamoxifen reduces the annual breast cancer death rate by 31% while the breast cancer mortality rate throughout the next 15 years would be approximately halved (Ref. 4). However, patients' responses to tamoxifen vary. In total, 30–50% of patients with adjuvant tamoxifen therapy experience relapse and subsequently die of the disease (Refs 2, 4). This evidence concerns the gene ESR1 and breast cancer.