Chromosomal passenger protein that seems to be required for chromosome segregation and the onset of cytokinesis during mitosis. Plays a key role in the abscission of daughter cells at the end of cytokinesis and in the establishment or maintenance of a bipolar spindle. Interacts with aurK. During mitosis, found in the spindle midzone and poles. During cytokinesis, found in the cleavage furrow. During prometaphase, found concentrated adjacent to the condensed chromosome. During metaphase, found concentrated in the middle of the spindle surrounded by chromosomes. After the onset of anaphase, moved to the spindle midzone and spindle poles where it remains through late telophase. After initiation of the cleavage furrow, found increasingly concentrated at the furrow area, especially at the cortex area. By the abscission stage of cytokinesis, found highly concentrated in the thin cytoplasmic bridge connecting the two daughter cells. Null cells divide much more slowly than wild type, have a high percentage of mutant cells, become multinucleate and have very enlarged nuclei. Belongs to the INCENP family.