The eukaryotic pre-RC is the most complex and highly regulated pre-RC. In most eukaryotes it is composed of six ORC proteins (ORC1-6), Cdc6, Cdt1, and a heterohexamer of the six MCM proteins (MCM2-7). The MCM heterohexamer arguably arose via MCM gene duplication events and subsequent divergent evolution. The pre-RC of "Schizosaccharomyces pombe" ("S. pombe") is notably different from that of other eukaryotes; Cdc6 is replaced by the homologous Cdc18 protein. Sap1 is also included in the "S. pombe" pre-RC because it is required for Cdc18 binding. The pre-RC of "Xenopus laevis" ("X. laevis") also has an additional protein, MCM9, which helps load the MCM heterohexamer onto the origin of replication. The structure of the ORC, MCM, as well as the intermediate ORC-Cdc6-Cdt1-Mcm2-7 (OCCM) complex has been resolved.