Bub1 was initially identified as the mammalian homolog to the yeast protein through a cDNA screen. Bub1 encodes a kinase involved in the formation of the spindle checkpoint, an important mechanism that ensures high fidelity mitotic chromosome segregation. It is thought to be required for assembly of a functional inner centromere, sister chromatid cohesion via targeting of the Shugoshim protein and metaphase congression. Bub1 functions by phosphorylating cdc20, a member of the mitotic checkpoint complex and activating the spindle checkpoint. A related protein kinase Bub3 interacts with Bub1 and targets it to kinetochores prior to chromosome alignment. Mutations in bub1 have been associated with aneuploidy and several forms of cancer.