Performs two crucial functions during mitosis: it is essential for spindle-assembly checkpoint signaling and for correct chromosome alignment. Directly links spindle checkpoint proteins BUB1 and BUB1B to kinetochores. Part of the MIS12 complex; which may be fundamental for kinetochore formation and proper chromosome segregation during mitosis. Acts in coordination with CENPK to recruit the NDC80 complex to the outer kinetochore (By similarity). {ECO:0000250}.