CDK4 and cancer: Moreover, Cdk4 and Cdk6 have been shown to differ functionally from one another in several respects [16-19] and Cdk4 has been reported to be closely co-expressed with Cdk1 in a wide range of human cancers in-vitro and in malignant melanoma in the clinic but not in normal diploid fibroblasts or keratinocytes [20].