Invasive oestrogen receptor (ER)-positive (ER+) breast cancers were reduced by 35% (HR = 0.65 (0.53–0.80), P < 0.0001), but no effect was seen for invasive ER-negative (ER-) breast cancers (HR = 1.06 (0.71–1.58), P = 0.8). This evidence concerns the gene ESR1 and breast cancer.