ESR1 and breast cancer: The AR is more commonly expressed in breast epithelial cells than the ER, and while co-localization of AR and ER can occur, more often AR is expressed without ER.21 ER infrequently co-localizes with Ki67 in normal breast tissue, though this does often occur in malignant tissue.34,35 Prior analyses in this case–control study indicated that women with high Ki67 and low ER expression in normal TDLUs experienced higher breast cancer risk.29 Our findings indicated no heterogeneity for the association between Ki67 expression and breast cancer risk with respect to AR expression.