CDK4-deficient mice were resistant to skin tumour development induced by MYC[15], whereas mice lacking cyclin D1 expression and consequently lacking CDK4 activation still developed mammary tumours induced by MYC activation[51], strongly arguing that the requirement for CDK4 activity in MYC-induced tumorigenesis is affected by cellular context and tissue type. This evidence concerns the gene CCND1 and skin neoplasm.