The major function of the mitotic spindle is to ensure that when cells divide each daughter faithfully receives one copy of each chromosome. For this to happen, spindle microtubules first attach to kinetochores, the proteinaceous complexes assembled at the surface of the centromeres of each chromosome. Microtubule motors at the kinetochores (along with others, named chromokinesins, bound to the chromosome arms) power chromosome congression to an aligned configuration, called metaphase. Subsequently, the main mitotic business ensues: the degradation of the glue holding duplicated sister chromosome pairs together, and the kinetochore-directed movement of one copy of each chromosome to each spindle pole, a process referred to as anaphase.

Fidelity is key: chromosomes lost along the way generate aneuploidy, an event generally lethal during development and a feature of the aberrant growth regulation associated with tumor progression. Faithful transmission of chromosomes is mediated by a spindle assembly checkpoint (sometimes called the mitotic checkpoint) that acts to prevent anaphase until all kinetochores have successfully captured one or more spindle microtubules, thus ensuring that no chromosome is left behind, either at mitosis or meiosis. The spindle checkpoint represents a highly conserved mechanism at work in many cell types from single-celled organisms through metazoans.