CYP19A1 and breast cancer: With an increasing body of clinical studies proving that endocrine therapy drugs, such as tamoxifen and, subsequently, emergent fulvestrant and aromatase inhibitors (AIs), have preeminent curative effects on HR-positive breast cancers, a consensus has been reached that endocrinotherapy is preferred to chemotherapy for such patients because the former treatment is less toxic and easier tolerated than the latter one.