Once the Mcm2-7 complex has been assembled onto the origin of replication, the next step is the assembly of Cdc45, an essential replication protein, in late G1. The assembly of Cdc45 onto origins of replication forms a complex distinct from the pre-replicative complex, sometimes called the pre-initiation complex. The assembly of Cdc45 onto origins correlates with the time of initiation. Like the Mcm2-7 proteins, Cdc45 binds specifically to origins in the G1 phase of the cell cycle and then to non-origin DNA during S phase and is therefore thought to travel with the replication fork. Indeed, S. cerevisiae Cdc45 is required for DNA replication elongation as well as replication initiation. Cdc45 is required for the association of alpha DNA polymerase:primase with chromatin. Based on this observation and the observation that in S. cerevisiae, cCdc45 has been found in large complexes with some components of Mcm2-7 complex, it has been suggested that Cdc45 plays a scaffolding role at the replication fork, coupling Pol-alpha:primase to the replication fork through the helicase. Association of Cdc45 with origin DNA is regulated in the cell cycle and its association is dependent on the activity of cyclin-dependent kinases but not the Cdc7/Dbf4 kinase. In Xenopus egg extracts, association of Cdc45 with chromatin is dependent on Xmus101. TopBP1, the human homolog of Xmus1, is essential for DNA replication and interacts with DNA polymerase epsilon, one of the polymerases involved in replicating the genome. TopBP1 homologs have been found in S. cerevisiae and S. pombe. Sld3, an additional protein required for Cdc45 association with chromatin in S. cerevisiae and S. pombe, has no known human homolog.