Required for the execution of the mitotic checkpoint which monitors the process of kinetochore-spindle attachment and delays the onset of anaphase when this process is not complete. It inhibits the activity of the anaphase promoting complex by sequestering CDC20 until all chromosomes are aligned at the metaphase plate. Required for anchoring MAD2 to the nuclear envelope. Homodimer. Part of the mitotic checkpoint complex (MCC). Interacts with MAD2 and NUA. The nucleus envelope association requires the presence of NUA. Belongs to the MAD1 family.