These concerns have been fuelled by RNA interference and knockout mice studies, indicating that functional redundancy may exist between different Cdks, with the loss of Cdk2 alone failing to inhibit tumour cell proliferation (Ortega et al, 2003; Tetsu and McCormick, 2003), and the loss of both Cdk4 and Cdk6 not preventing cell cycling (Malumbres et al, 2004). This evidence concerns the gene CDK4 and neoplasm.