Palbociclib, a CDK4/6 inhibitor, has received accelerated approval in the U.S by the Food and Drug Administration (FDA) for the first line treatment of HR+/HER2neg advanced breast cancer in combination with the hormonal treatment letrozole [6] and, more recently, for the treatment of endocrine pre-treated patients in combination with the anti-estrogen fulvestrant, given the results of three pivotal randomized clinical trials, the PALOMA-1, PALOMA-2 and PALOMA-3 [7-10]. This evidence concerns the gene CDK4 and breast cancer.