BubR1 is cell cycle checkpoint kinase and component of the spindle checkpoint machinery. BubR1 and other spindle checkpoint proteins ensure that segregation does not proceed until all replicated chromatids are associated with a functional bipolar spindle. BubR1 is part of the mitotic checkpoint complex that also includes Bub3, Mad2, and Cdc20. In this trimer BubR1 functions to inhibit the anaphase promoting complex (APC) that promotes chromosome segregation. BubR1 is suggested to be a paralog of Bub1.