Patients whose cancer cells have a receptor for the hormones oestrogen and/or progesterone are usually treated with endocrine therapy (ET), which uses an aromatase inhibitor to reduce the body’s production of oestrogen and a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor to block the growth of cancer cells [96]. The gene discussed is CYP19A1; the disease is cancer.