Component of the spindle-assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at the metaphase plate. Required for the execution of the mitotic checkpoint which monitors the process of kinetochore-spindle attachment and inhibits the activity of the anaphase promoting complex until all chromosomes are aligned at the metaphase plate (By similarity). Interacts with cdc20. The protein has two highly different native conformations, an inactive open conformation that cannot bind cdc20 and that predominates in cytosolic monomers, and an active closed conformation. Belongs to the MAD2 family. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.