Fig. 10. The spindle checkpoint is composed of two arms, both of which promote and/or maintain BubR1 binding to the APC/C. The Bub1-dependent arm monitors microtubule attachment at kinetochores and is therefore activated by nocodazole. By contrast, the aurora-dependent arm monitors biorientation and is therefore activated by both nocodazole and taxol. Both arms converge on other checkpoint proteins including BubR1, Bub3, Mad2 and Cdc20, promoting their association with the APC/C and thereby preventing the metaphase to anaphase transition until all the chromosomes are correctly aligned on the spindle.