CYP19A1 and breast cancer: The mainstay first-line treatment for the estimated 7500 post-menopausal women with advanced breast cancer in the UK are endocrine therapy with steroidal or non-steroidal aromatase inhibitors.5 Recent clinical studies have demonstrated the addition of cyclin-dependent kinase 4 and 6 inhibitors (CDK 4/6) inhibitors (such as ribociclib and palbociclib) to aromatase inhibitors (such as letrozole) provides rapid clinical improvement in patients with measurable disease.