Acts as a component of the essential kinetochore- associated NDC80 complex; which is required for chromosome segregation and spindle checkpoint activity. Required for kinetochore integrity and the organization of stable microtubule binding sites in the outer plate of the kinetochore. {ECO:0000269|PubMed:12438418; ECO:0000269|PubMed:14654001; ECO:0000269|PubMed:15062103; ECO:0000269|PubMed:15235793; ECO:0000269|PubMed:15239953; ECO:0000269|PubMed:15548592; ECO:0000269|PubMed:17535814}.