sentences:  The loading of Cdc45 onto chromatin is critical for loading other various replication proteins, including DNA polymerase α, DNA polymerase ε, replication protein A (RPA) and proliferating cell nuclear antigen (PCNA) onto chromatin.Within a "Xenopus" nucleus-free system, it has been demonstrated that Cdc45 is required for the unwinding of plasmid DNA.The "Xenopus" nucleus-free system also demonstrates that DNA unwinding and tight RPA binding to chromatin occurs only in the presence of Cdc45.