CDK4 and cancer: Cdk4 classically acts as a kinase phosphorylating the Rb protein and releasing E2F when it is activated by being conjoined with cyclin D1 to form a holoenzyme26 Experiments in mice in which the wild‐type CDK4 gene on its own has been knocked down leaving normal Cyclin D activity, have, however, shown that without Cdk4 the induction of cancer by chemical carcinogenesis with 7,12‐dimethyl‐benz[a]anthracene applied to mouse skin, followed by the tumour promoter 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) was severely blocked.