The function of KNSTRN (kinetochore localized astrin (SPAG5) binding protein, ENSG00000128944) is as follows. Essential component of the mitotic spindle required for faithful chromosome segregation and progression into anaphase (PubMed:19667759). Promotes the metaphase-to-anaphase transition and is required for chromosome alignment, normal timing of sister chromatid segregation, and maintenance of spindle pole architecture (PubMed:19667759, PubMed:22110139). The astrin (SPAG5)-kinastrin (SKAP) complex promotes stable microtubule-kinetochore attachments (PubMed:21402792). Required for kinetochore oscillations and dynamics of microtubule plus-ends during live cell mitosis, possibly by forming a link between spindle microtubule plus-ends and mitotic chromosomes to achieve faithful cell division (PubMed:23035123). May be involved in UV-induced apoptosis via its interaction with PRPF19; however, these results need additional evidences (PubMed:24718257). {ECO:0000269|PubMed:19667759, ECO:0000269|PubMed:21402792, ECO:0000269|PubMed:22110139, ECO:0000269|PubMed:23035123, ECO:0000305|PubMed:24718257}.