Checkpoint: A surveillance system responsible for monitoring the proper completion of an event within a cell. Cells contain a number of such systems. When a checkpoint system detects the failure of an event, it signals and calls for the inhibition of events downstream to the failure. These surveillance systems are termed checkpoints because they represent stages in the cell cycle at which the cell checks to be certain it is okay to proceed. Checkpoints control the order and timing of cell cycle transitions and ensure that critical events, such as DNA replication and chromosome segregation, are completed with high fidelity.