ESR1 and breast cancer: Genetically predicted higher IGF‐1 levels was significantly associated with higher odds of breast cancer in the BCAC (OR, 1.08; 95% CI 1.02‐1.13; P = 4.4 × 10−3), with similar estimates for estrogen‐receptor positive (OR, 1.07; 95% CI 1.01‐1.13; P = .02) and estrogen‐receptor negative (OR, 1.08; 95% CI 1.00‐1.16; P = .04) breast tumors.