In response to DNA damage, Saccharomyces cerevisiae cells arrest the cell cycle in the G2 phase. Arrest is defective in rad9 mutants; rad9 cells divide and die without repairing the damage. Several cell cycle mutants that are defective in DNA replication arrest in G2 at the restrictive temperature; this arrest is due to the RAD9 control function. Thus RAD9 is responsible for the fact that mitosis is normally dependent upon DNA replication, a function we term a ‘checkpoint’. Four additional genes have been identified that are also components of the RAD9 checkpoint.