CYP19A1 and breast cancer: Endocrine therapy (ET) with tamoxifen or aromatase inhibitor (AI) (letrozole, anastrozole, exemestane) has been recommended for hormone receptor positive (HR+) breast cancer patients after BCS plus RT to reduce the risk of contralateral breast cancer (CBC) and ipsilateral breast tumor recurrence by National Comprehensive Cancer Network (Allred et al., 2012; Forbes et al., 2016; Ganz et al., 2016).