ERBB2 and breast cancer: When combining adult BMI and comparative body size at age 10 in a multivariable MR model examining overall and luminal B HER2 negative-like breast cancer risk, the direct effect of adult BMI on breast cancer risk was attenuated for overall and luminal BHER2 negative-like breast cancer risk (overall breast cancer risk OR:1.09, 95% CI:0.99–1.20, p = 0.085) but the direct effect of comparative body size at age 10 was retained for overall and luminal B HER2negative-like breast cancer risk (overall breast cancer risk OR:0.56, 95% CI:0.46–0.67, p = 5.04x10−10)(Table S8).