Subunit of the spindle-assembly checkpoint complex; involved in delaying anaphase onset in cells with defects in mitotic spindle assembly; pseudosubstrate inhibitor of APC(Cdc20), the anaphase promoting complex involved in Pds1p (securin) turnover; MAD3 has a highly diverged paralog, BUB1, that arose from the whole genome duplication; N-terminal TPR domain-containing region is homologous with the N-terminal region of Bub1p