Some of the spindle checkpoint components are evolutionarily conserved from yeast to human. They include mitotic arrest deficiency 1 (MAD1), MAD2, MAD3/BUBR1, budding uninhibited by benomyl 1 (BUB1), BUB3, and monopolar spindle 1 (MPS1). MAD1 interacts with several checkpoint proteins, forming a complex with MAD2 and recruiting MAD2 to unattached kinetochores. MAD1 and Cdc20 can bind MAD2, but these two bindings are exclusive. They share a 10-residue MAD2-binding motif. It has been suggested that MAD1 acts as a positive regulator and a competitive inhibitor of the MAD2-Cdc20 complex.Synonyms: MAD2, Mitotic spindle assembly checkpoint protein MAD2A