Consistent with this idea, all three of the proteins in the complex are tumor suppressors, meaning that mutations in them make tumor formation more likely. The researchers confirmed that the mitotic stopwatch was frequently defective in tumor samples.

It seems possible that this is not the only 'stopwatch' mechanism, if some/most tumor samples don't have a defective version of it. If there were two or more for redundancy, then all of them being defective would make tumors more likely.