CDK4 and melanoma: CDK4 expression levels may not vary in different cell lines or patients, but may be subject to the R24C mutation (which prevents the p16INK4 binding), or p16INK4 may be deleted, or cyclin D may be overexpressed, all of which lead to kinase hyperactivation in melanoma and other cancers [20,21,22,23,24] (Figure 1).