Mitotic checkpoint protein BUB3, also known as BUB3, is a conserved component of the mitotic spindle assembly complex (MCC). The encoded protein contains four WD repeat domains and has sequence similarity with the yeast BUB3 protein. BUB3 is essential for the kinetochore localization of BUB1 and BUBR1. BUB3 is involved in the essential spindle checkpoint pathway that operates during early embryogenesis. In addition, BUB3 plays a role in regulating the establishment of correct kinetochore-microtubule attachments. Recombinant human BUB3 protein, fused to His-tag at N-terminus, was expressed in E.coli.