CDK4 and breast cancer: Stemming from several preclinical and clinical studies showing that CDK4/6i act synergistically with ET and can overcome ET resistance [29,30,31,32], a series of randomized trials, namely PALOMA [33,34,35,36,37], MONALEESA [31,38,39,40,41,42] and MONARCH [43,44,45,46,47], showed that CDK4/6i increase progression-free survival (PFS) compared with ET alone in HR+/HER2− BC.