Eukaryotes typically have multiple origins of replication; at least one per chromosome. "Saccharomyces cerevisiae" ("S. cerevisiae") is the only known eukaryote with a defined initiation sequence TTTTTATG/ATTTA/T. This initiation sequence is recognized by ORC1-5. ORC6 is not known to bind DNA in "S. cerevisiae". Initiation sequences in "S. pombe" and higher eukaryotes are not well defined. However, the initiation sequences are generally either AT-rich or exhibit bent or curved DNA topology. The ORC4 protein is known to bind the AT-rich portion of the origin of replication in "S. pombe" using AT hook motifs. The mechanism of origin recognition in higher eukaryotes is not well understood but it is thought that the ORC1-6 proteins depend on unusual DNA topology for binding.