Acts as a component of the essential kinetochore-associated NDC80 complex, which is required for chromosome segregation and spindle checkpoint activity (PubMed:14699129, PubMed:14738735). Required for kinetochore integrity and the organization of stable microtubule binding sites in the outer plate of the kinetochore (PubMed:14738735, PubMed:14699129). The NDC80 complex synergistically enhances the affinity of the SKA1 complex for microtubules and may allow the NDC80 complex to track depolymerizing microtubules (PubMed:23085020). Component of the NDC80 complex, which consists of NDC80/HEC1, CDCA1, SPBC24 and SPBC25. The NDC80 complex is formed by two subcomplexes composed of NDC80/HEC1-CDCA1 and SPBC24-SPBC25. Each subcomplex is formed by parallel interactions through the coiled-coil domains of individual subunits. Formation of a tetrameric complex is mediated by interactions between the C-terminal regions of both subunits of the NDC80/HEC1-CDCA1 subcomplex and the N-terminal regions of both subunits of the SPBC24-SPBC25 complex. The tetrameric NDC80 complex has an elongated rod-like structure with globular domains at either end. Localizes to kinetochores from late prophase to anaphase (PubMed:14738735). Localizes specifically to the outer plate of the kinetochore (PubMed:14738735, PubMed:14699129). Belongs to the SPC25 family.