In advanced/metastatic breast cancer, CDK4/6 inhibitors (CDK4/6i) are approved in the UK for use in HR+ HER2- breast cancer, either in previously untreated patients or following endocrine therapy, in combination with fulvestrant or aromatase inhibitors [108, 109, 111, 112, 115]. The gene discussed is CYP19A1; the disease is breast carcinoma.