Component of the aurora kinase complex, also called chromosomal passenger complex (CPC), essential for chromosome segregation and metaphase chromosome alignment. Mediates the SLI15-BIR1 interaction within the CPC. Component of the aurora kinase complex composed of at least BIR1, BNL1, IPL1 and SLI15. In G1, localizes along nuclear microtubules and at spindle poles. In late S phase to metaphase, is seen as a few small nuclear foci and also localizes diffusely within the nucleus. Also found along the spindle in early anaphase, at the spindle midzone in late anaphase, and finally concentrated at the plus ends of depolymerizing microtubules before mitotic exit. Belongs to the borealin family.