Nek2 promotes centrosome separation at mitotic onset. G2 cells contain a duplicated centrosome that consists of two pairs of centrioles (yellow cylinders) surrounded by pericentriolar material (grey cloud). It is proposed that the two centrosomes lie in close proximity as a result of a proteinaceous linker that connects the proximal ends of the parental centrioles. This structure contains at least two proteins, C-Nap1 (blue disc) and rootletin (red fibres). At this time, the Nek2 kinase, which exists as a stable homodimer, is inhibited by the protein phosphatase, PP1. Upon entry into mitosis, PP1 itself is inhibited as a result of binding of the Inhibitor-2 protein (Inh-2), an interaction that may be stimulated by Cdk1. The consequence is that Nek2 kinase is activated leading to phosphorylation and displacement of C-Nap1 and rootletin from the centrosome. The two disconnected pairs of centrioles can then be driven apart to form the two poles of the emerging mitotic spindle.