Purpose of excess MCMs: to license dormant origins as a backup mechanism for stress: (a) prior to S phase, the MCMs license main origins as well as many dormant origins which remain prevented to be fired by Chk1 signalling. At the time of stress, when replication forks get stalled, the backup origins get activated and continue the replication process; (b) in case of a limited licensing condition, at the time of stress there is no backup to retrieve the replication process; hence, the cells cannot withstand the stress (adapted from [71]).