Component of the chromosomal passenger complex (CPC), a complex that acts as a key regulator of embryonic mitosis. The CPC complex has essential functions at the centromere for ensuring sister chromatid cohesion, recruitment of the CPC to kinetochores, and chromosome alignment and segregation. There is no function in meiotic histone phosphorylation or spindle formation. Component of the CPC complex. Localizes on chromosome arms and inner centromeres from prophase through metaphase and then transferring to the spindle midzone and midbody from anaphase through cytokinesis. Colocalizes with and is required for the correct localization of the CPC components Aurora B kinase and Incenp in mitotic cells. Is absent from spermatocytes undergoing meiosis. Ubiquitously expressed in the early embryo. Expression is restricted to the ventral nerve cord and brain during later embryonic stages. Flies show multiple mitotic defects, including multipolar spindles that result in large polyploid cells and often in delayed apoptosis. The developmental consequences of these defects include cell-autonomous and non-autonomous defects in cell-type specification and tissue architecture. Flies also display a drastic decrease in histone H3 'Ser-10' phosphorylation, suggesting that the CPC complex mediates phosphorylation of 'Ser-10' of histone H3. Belongs to the borealin family.