ESR1 and breast cancer: Results from the MONALEESA-2 study demonstrated that dual blockade of the CDK4/6 and estrogen receptor pathways improves clinical outcomes in patients with HR+, HER2− advanced breast cancer; first-line ribociclib plus letrozole significantly prolonged PFS compared with letrozole alone (hazard ratio: 0.556; 95% CI 0.429–0.720; p = 3.29 × 10−6) [13].