Current standard first-line endocrine treatment for post-menopausal HR+/HER2- metastatic breast cancer patients implies the administration of an aromatase inhibitor with a CDK4/6 inhibitor, abemaciclib, palbociclib or ribociclib, based on the results of three recent phase III clinical trials showing superior PFS as compared with aromatase inhibitor clone. Here, CYP19A1 is linked to breast cancer.