CDK4 and oral cavity squamous cell carcinoma: The treatment of oral squamous cell carcinoma cells, Cal27, GNM, and WSU-HN6, with a H2S donor, NaHS, significantly downregulated cell cycle regulatory genes, RPA70 and RB1, and upregulated proliferating cell nuclear antigen and cyclin-dependent kinase 4 (CDK4), resulting in cell proliferation (Ma et al., 2015).