The mitotic checkpoint protein Bub3 is involved with the essential spindle checkpoint pathway which operates during early embryogenesis. Bub3 is important during G2 and early mitosis stages, permitting entry into mitosis depending upon the assembly state of microtubules, thus preventing premature sister chromatid separation, mis-segregation and aneuploidy. Bub3 contains four WD repeat domains and is required for the kinetochore localization of Bub1, a related kinase that is necessary for spindle assembly checkpoint function. Bub1 is able to autophosphorylate and can catalyze the phosphorylation of Bub3. Both Bub1 and Bub3 are mutually dependent for function. Altered Bub expression levels may significantly impair mitotic checkpoint function and is associated with tumor cell proliferation.