CDK4 and neoplasm: By using 2D-gel electrophoresis to separate the phosphorylated forms of CDK4 and CDK6 bound to their regulatory partners, we have identified the activating T172 phosphorylation as the last, separately regulated, critical step of CDK4 activation in such tumor cells [3], [35]–[37] and in various normal cells [31], [38], [39].