CDK4 and breast cancer: ,2,3 CDK4/6 inhibitor (CDKI) combined with endocrine therapy is recommended as the first line regimen for patients with hormone receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer (ABC) according to the current guidelines,4,5 since the series of PALOMA,6,7,8 MONALEESA,9,10,11 and MONARCH12,13 clinical trials have demonstrated that the addition of endocrine therapy to CDKI achieved a substantially incremental improvement in progression-free survival (PFS) for these patients.