In the presence of free kinetochores, cells do not undergo anaphase or exit mitosis, suggesting that the spindle checkpoint pathway blocks both the metaphase!anaphase and anaphase!telophase transitions. Components of the spindle checkpoint were originally defined in budding yeast, but homologues were later identified in higher eukaryotes, demonstrating that the pathway is
conserved. Two independent, but similar, screens (Hoyt et al., 1991; Li andMurray, 1991) for yeast mutants that do not arrest in mitosis following drug treatment identified six
proteins: Bub1–3 (budding uninhibited by benzimidazole) and Mad1–3 (mitosis arrest deficient). Some of these proteins are localized at the kinetochore and some at spindle poles, the two locations generating signals for the spindle checkpoint.