Required for initiation of chromosomal DNA replication. Binds to chromatin in a mutually dependent manner with the GINS DNA replication complex. Interacts with topbp1 (PubMed:12743046). Component of the CMG helicase complex, composed of the mcm2-7 complex, the GINS complex and cdc45 (PubMed:30979826, PubMed:30842657). Associates with chromatin. Belongs to the CDC45 family.