In higher animal cells, duplication of centrosomes is triggered by CDK2/cyclin E-mediated phosphorylation. Nucleophosmin (NPM)/B23, a multifunctional protein, has recently been identified as one of the substrates of CDK2/cyclin E in centrosome duplication. Centrosome-bound NPM/B23 dissociates from centrosome upon phosphorylation by CDK2/cyclin E, which in turn triggers initiation of centriole duplication. Duplicated centrosomes remain free of NPM/B23 till mitosis. When the nuclear membrane breaks down during mitosis, NPM/B23 re-localizes to centrosomes. Upon cytokinesis, each daughter cell receives one centrosome bound by NPM/B23, which again dissociates from the centrosome upon exposure to CDK2/cyclin E at mid-late G1 phase of the next cell cycle. Thus, NPM/B23 would constitute one of the licensing systems for centrosome duplication, ensuring the coordination of centrosome and DNA duplication, which limiting duplication once per cell cycle.