NEK9 functions as a homodimer and becomes catalytically active in mitosis through phosphorylation (Roig et al. 2002). While threonine T333 of NEK9 is phosphorylated in both interphase and mitotic cells (Roig et al. 2005, Bertran et al. 2011), serine residues S29, S750 and S869 of NEK9 are phosphorylated only in mitotic cells. S29, S750 and S869 sites are proline directed and match the CDK1 consensus sequence (Bertran et al. 2011). CDK1:CCNB complex was shown to phosphorylate NEK9 in vitro (Roig et al. 2002).