In the randomized, open-label, phase II PALOMA-1/TRIO-18 trial, the safety and efficacy of palbociclib in combination with the aromatase inhibitor letrozole was compared with letrozole alone as first-line treatment of women with ER-positive, HER2-negative (ER+/HER2–), advanced breast cancer [3]. Here, CYP19A1 is linked to breast cancer.