Part of the checkpoint control that prevents mitosis from occurring until S phase is completed. Plays a key role in coupling S phase to start and mitosis. Acts at the initiation of DNA replication and plays a major role in controlling the onset of S-phase. Together with orc1, involved in the maintenance of replication forks and activation of cds1-dependent S-phase checkpoint. Together with orc2, plays a role in preventing DNA rereplication and resulting genetic instability (PubMed:11486016). Interacts with the origin recognition complex (ORC) and is recruited to the ars1 origin of replication. Interacts with cdt1, pop1 and pop2. Ubiquitinated by pop1 and pop2 and targeted to the 26S proteasome for degradation. Belongs to the CDC6/cdc18 family.