There are various mechanisms of drug resistance in breast cancer, such as endocrine resistance mechanisms (resistance to tamoxifen or aromatase inhibitors), chemotherapy resistance mechanisms (drug efflux), DNA damage repair (DDR) mechanisms, tumor microenvironment (TME) and tumor cell crosstalk mechanisms, targeted therapy mechanisms, etc (Zeng et al., 2018). The gene discussed is CYP19A1; the disease is neoplasm.