CDK4 and cancer: The advent of the first CDK4/6 specific inhibitor (i.e. PD0332991, then renamed Palbociclib) has drastically changed this view when preclinical data demonstrated that this compound was able to block cancer cells in G1 phase of the cell cycle, inhibiting CDK4/6-cyclin D complexes with exquisite selectivity and displaying very promising antitumor activities in mice (3–6).