Regulator of DNA replication and S/M and G2/M checkpoints. Regulates the triggering of DNA replication initiation via its interaction with topbp1 by participating in cdk2-mediated loading of cdc45l onto replication origins. Required for the transition from pre-replication complex (pre-RC) to pre-initiation complex (pre-IC). Required to prevent mitotic entry after treatment with ionizing radiation. Interacts with topbp1 (via BRCT domains); interaction takes place in a cdk2-dependent manner (By similarity). Component of the replisome complex (By similarity). Associates with chromatin. Embryonic lethality, due to impaired S-phase progression and disruption of the S/M checkpoint, leading to premature mitotic entry and mitotic catastrophe. Belongs to the treslin family.