<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>Phases or points in the cell cycle at which a cell is prevented from progressing into the next phase of the cycle unless certain necessary and sufficient cellular conditions exist. The checkpoints help maintain the proper order and timing of cell transition events, especially important for DNA replication and chromosome segregation.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>