This family is involved in the cell-cycle surveillance mechanism called the spindle checkpoint. This mechanism monitors the proper bipolar attachment of sister chromatids to spindle microtubules and ensures the fidelity of chromosome segregation during mitosis. A key player in mitosis is Mad2, and Mad2 exhibits an unusual two-state behaviour. A Mad1-Mad2 core complex recruits cytosolic Mad2 to kinetochores through Mad2 dimerization and converts Mad2 to a conformer amenable to Cdc20 binding. p31comet inactivates the checkpoint by binding to Mad1- or Cdc20-bound Mad2 in such a way as to stop Mad2 activation and to promote the dissociation of the Mad2-Cdc20 complex.