In the PERTAIN trial [54], it was hypothesized that pertuzumab, trastuzumab, and an endocrine-therapy (aromatase inhibitor) may offer additional benefits compared with trastuzumab plus an aromatase inhibitor for HER2–positive and hormone receptor–positive MBC or locally advanced breast cancer (LABC) in first line. Here, CYP19A1 is linked to breast cancer.