The link between GM and BC initially stemmed from epidemiological studies, which initiated inquiries into the influence of antibiotic use on BC.[11] By reducing estrogen metabolism via the GM, it is possible to lower circulating estrogen and thereby decrease the risk of developing estrogen receptor-positive BC.[12,13] However, epidemiological studies might encounter certain limitations, for instance, measurement errors, uncontrolled confounding factors, and reverse causality. This evidence concerns the gene ESR1 and breast cancer.