Cell division is a tightly regulated process that ensures the segregation of chromosomes into daughter cells. Essential to this regulation is the modification of cell cycle components by reversible phosphorylation. Ipl1 and aurora are two related kinases isolated from S.cerevisiae and Drosophila, respectively. Inactivation of these kinases results in abnormal chromosome segregation and disruption of the centrosome. A structurally and functionally similar kinase, IAK1 (Ipl1- and Aurora-related kinase 1), is a regulator of mammalian chromosome segregation. Although IAK1 may be present in the cytoplasm, it is detected on the centrosome following duplication and also associates with the spindle microtubules from metaphase through cell division. Expression of IAK1 is stringently regulated during the cell cycle. Both mRNA and protein are initially expressed in S-phase, are elevated during M-phase, and are undetectable following completion of mitosis. Increasing evidence suggests that IAK1 belongs to a novel subfamily of the ser/thr kinase superfamily. Although mutational analysis of IAK1 will directly determine its function, it appears to be a key player in the control of cell division.