Using different types of genetic studies, it has been established that diverse kinds of defects are able to activate the SAC: spindle depolymerization, the presence of dicentric chromosomes (with two centromeres), centromeres segregating in an aberrant way, defects in the spindle pole bodies in "S. cerevisiae", defects in the kinetochore proteins, mutations in the centromeric DNA or defects in the molecular motors active during mitosis. A summary of these observations can be found in the article from Hardwick and collaborators in 1999.