The two mitotic centrosomes direct spindle bipolarity to maintain euploidy. Centrosome amplification-the acquisition of > or =3 centrosomes-generates multipolar mitoses, aneuploidy, and chromosome instability to promote cancer biogenesis. While much evidence suggests that Cdk2 is the major conductor of the centrosome cycle and that it mediates centrosome amplification induced by various altered tumor suppressors, the role played by Cdk4 in a normal or deregulated centrosome cycle is unknown. Using a gene knockout approach, we report that Cdk2 and Cdk4 are critical to the centrosome cycle, since centrosome separation and duplication are premature in Cdk2(-)(/)(-) mouse embryonic fibroblasts (MEFs) and are compromised in Cdk4(-)(/)(-) MEFs. Additionally, ablation of Cdk4 or Cdk2 abrogates centrosome amplification and chromosome instability in p53-null MEFs. Absence of Cdk2 or Cdk4 prevents centrosome amplification by abrogating excessive centriole duplication. Furthermore, hyperactive Cdk2 and Cdk4 deregulate the licensing of the centrosome duplication cycle in p53-null cells by hyperphosphorylating nucleophosmin (NPM) at Thr199, as evidenced by observations that ablation of Cdk2, Cdk4, or both Cdk2 and Cdk4 abrogates that excessive phosphorylation. Since a mutant form of NPM lacking the G(1) Cdk phosphorylation site (NPM(T199A)) prevents centrosome amplification to the same extent as ablation of Cdk2 or Cdk4, we conclude that the Cdk2/Cdk4/NPM pathway is a major guardian of centrosome dysfunction and genomic integrity.