Central component of the spindle assembly checkpoint. Thought to recruit MAD2 to unattached kinetochores. During checkpoint activity, MAD2 is relayed from the MAD1-MAD2 complex to the mitotic checkpoint complex (MCC). The formation of a MAD1-BUB1-BUB3 complex seems to be required for the spindle checkpoint mechanism. Forms a stable heteromer with MAD2 throughout the cell cycle. Part of complex consisting of MAD1, BUB1 and BUB3 after activation of spindle checkpoint. Becomes hyperphosphorylated when wild-type cells are arrested in mitosis. Phosphorylated by MPS1. Present with 656 molecules/cell in log phase SD medium. Belongs to the MAD1 family.