Involved in mitotic spindle assembly checkpoint signaling, a process that delays anaphase until chromosomes are bioriented on the spindle, and in the repair of incorrect mitotic kinetochore-spindle microtubule attachments (PubMed:18243099, PubMed:28441529, PubMed:29162720). Phosphorylates MAD1L1 to promote the mitotic spindle assembly checkpoint (PubMed:18243099, PubMed:29162720). Phosphorylates CDCA8/Borealin leading to enhanced AURKB activity at the kinetochore (PubMed:18243099). Phosphorylates SKA3 at 'Ser-34' leading to dissociation of the SKA complex from microtubules and destabilization of microtubule-kinetochore attachments (PubMed:28441529). Phosphorylates KNL1, KNTC1 and autophosphorylates (PubMed:28441529). Phosphorylates MCRS1 which enhances recruitment of KIF2A to the minus end of spindle microtubules and promotes chromosome alignment (PubMed:30785839). {ECO:0000269|PubMed:18243099, ECO:0000269|PubMed:28441529, ECO:0000269|PubMed:29162720, ECO:0000269|PubMed:30785839}. This is the function of TTK (TTK protein kinase, Ensembl gene identifier ENSG00000112742).