CDK4 and cancer: To explore the potential mechanism of CDK4 or CDK6 in regulating ABCB1 expression, cdk4 and cdk6 genes were knocked out by transfecting KB-C2 cancer cells with the all-in-one CRISPR/Cas9 plasmid harboring the sgRNA that targets either the cdk4 or cdk6 genes (Fig. 2).