Component of the chromosomal passenger complex (CPC), a complex that acts as a key regulator of chromosome segregation and cytokinesis during mitosis. The CPC complex has essential functions at the centromere in ensuring correct chromosome alignment and segregation. In the complex, it may be required to direct the Aurora B/air-2 to centromeric DNA. Component of the CPC complex which consists of icp-1; csc-1; bir-1 and air-2. Within the complex interacts with Aurora B/air-2, bir-1 and icp-1. Localizes on chromosome arms and inner centromeres from prophase through metaphase and then transferring to the central spindle from anaphase through cytokinesis. Belongs to the borealin family. Highly divergent.