Nbl1 is a subunit of the conserved CPC, the chromosomal passenger complex, which regulates mitotic chromosome segregation. In Fungi and Animalia, this complex consists of the kinase Aurora B/AIR-2/Ipl1p, INCENP/ICP-1/Sli15p, and Survivin/BIR-1/Bir1p. In Animalia, a fourth subunit (Borealin/Dasra/CSC-1) is required for targeting CPC to centromeres and central spindles. Nbl1 has been shown in budding yeast to be essential for viability, and for CPC localization, stability, integrity, and function. The N-terminus of Borealin is homologous to Nbl1. This family contains both Nbl1, and the N terminal region of Borealin.