It has been reported that G1 cell cycle arrest induced by pretreatment or concurrent treatment with highly selective CDK4/6 inhibitors prevented apoptotic triggering by different chemotherapeutics in RB1-positive breast cancer cells, melanoma cells and immortalized fibroblasts, thus suggesting that CDK4/6 inhibitors can antagonize chemotherapy [137, 139, 141]. This evidence concerns the gene CDK4 and breast cancer.