The protective effect of soy was significant in both estrogen receptor-positive and -negative breast cancer types, but the reduction in recurrence was stronger in the estrogen receptor-negative (HR = 0.64; 95% CI 0.44–0.94) compared with estrogen receptor-positive (HR = 0.81; 95% CI 0.63–1.04) breast cancer type. This evidence concerns the gene ESR1 and breast cancer.