CDK4 and breast cancer: Synergistic tumor-suppressing effects of CDK4/6 inhibitors and other selective inhibitors have been described in numerous clinical or preclinical studies, including CDK4/6i in combination with EGFR [97], PI3K [98] or FGFR inhibitors [99] in breast cancer xenografts, IGF1 inhibitors in pancreatic adenocarcinoma [100], MEK inhibitors in NRAS mutant melanoma [101], inhibitors of other targets such as MET/ERK [85], HER2 [102], MDM2/4 [73,103], eukaryotic initiation factor [104] and even pan-CDK inhibitors [105], etc. in various cancers.