CDK4 and neoplasm: Different rationales speak for targeting cell cycle proteins:(i)Aberrantly regulated CDKs (in melanoma 75 to 90% of tumors show mutations in the p16INK4A-cyclinD-CDK4/6-Rb pathway) (reviewed in [39]) allow for uncontrolled tumor growth overriding crucially important checkpoints, making these kinases very prominent drug targets.