The studies included predominantly women of European ancestry (approximately 84%), which limits the generalizability of the findings to other racial and ethnic groups.63 About 20% of the tumor subtype categorization was based on the immunohistochemistry of ER and PR, which may be inaccurate due to varying thresholds for positivity and interpretation criteria.64 Our study is also limited by residual confounding and potential misclassification due to self-reported exposure information, as well as by timing of exogenous hormone use in relation to breast cancer diagnosis. This evidence concerns the gene ESR1 and breast cancer.