Essential component of the mitotic checkpoint, which prevents cells from prematurely exiting mitosis. Required for the assembly of the dynein-dynactin and MAD1-MAD2 complexes onto kinetochores (PubMed:11146660, PubMed:11590237, PubMed:15824131). Its function related to the spindle assembly machinery is proposed to depend on its association in the mitotic RZZ complex. {ECO:0000269|PubMed:11146660, ECO:0000269|PubMed:11590237, ECO:0000269|PubMed:15824131, ECO:0000305}. This is the function of KNTC1 (kinetochore associated 1, Ensembl gene identifier ENSG00000184445).