Cdk4−/− breast tissue is indistinguishable from the normal mouse mammary gland; however, c-neu/ERBB-2-dependent tumor development is completely abrogated in the absence of CDK4 (Landis et al., 2006, Malumbres and Barbacid, 2006, Reddy et al., 2005). This evidence concerns the gene CDK4 and neoplasm.