Cry1 homozygous knockout mice, double Cry1/Cry2 knockout mice and Per2 mutant homozygous mutant mice exhibit increased rates of spontaneous tumours and tumours developing after genotoxic challenge (ionizing radiation).[17,21,22,23] The circadian pattern of expression of some genes coding for proteins involved in the control of the progression in the cell cycle (c-Myc, Cyclin D1, Cyclin A, Mdm-2) is grossly deregulated in mice with mutant Per2.[22,23]. This evidence concerns the gene MDM2 and neoplasm.