Licensing in the cell division cycle and on passage through G0. The small segment of chromosomal DNA that is shown encompasses three replication origins. On exit from mitosis (M), cells progress into G1 or into G0. In G1, the replication licensing system is activated (green), which causes MCM2-7 complexes (blue hexamers) to be loaded onto potential replication origins (i.e., origin licensing). The licensing system is turned off at the end of G1. Matson et al. (3) show that, in cycling cells, the licensing checkpoint is active and prevents cells from entering S phase if insufficient MCM2-7 have been loaded, but that in cells entering G1 from G0/quiescence, the licensing checkpoint is not active. During S phase, some MCM2-7 complexes are activated as helicases as origins fire (pink hexamers). MCM2-7 complexes are removed from replicated DNA, either during passive replication of unfired/dormant origins or at fork termination.