CDK4 and renal cell carcinoma: The molecular mechanism of action of ribociclib in RCC is through inhibiting the CDK4/6-cyclin D1-Rb pathway as shown by our findings that (1) ribociclib inhibits the phosphorylation of Rb at Ser807/811 and Ser795 (Figure 3(a)), (2) ribociclib decreases the transcriptional level of E2F target genes (Figure 3(b)), and (3) ribociclib does not affect Rb phosphorylation and mRNA level of E2F target genes in RCC cells that are resistant to ribociclib (Figure 3).