The roles of the cyclin-dependent kinases Cdk2, Cdk4 and Cdk6 and their complementary cyclin partners in moving cells from a quiescent state into active DNA synthesis are presently undergoing re-evaluation. Normal cell cycling now appears possible in the absence of any of these molecular controlling factors whilst certain cell-cycle control kinases, such as Cdk4, appear to be mandatory for cancer cell growth. Here, we describe a unique relationship between proteomic expression of Cdk1 and Cdk4 in human cancer cell lines and data from clinical malignant melanoma. The relationship was not present in normal diploid keratinocytes and fibroblasts. We suggest that the much tighter spread of Cdk1/Cdk4 ratios in human cancer cells compared to normal cells may selectively benefit the cancer cell and thus provide a potential novel anticancer target.