DNA replication is initiated by the Origin Recognition Complex (ORC). ORC is composed of six protein subunits and binds to the ARS Consensus Sequence (ACS) of Origins of Replication in an ATP-dependent manner. While ORC remains bound to origins throughout the cell cycle, the binding of ORC and other factors modulates local chromatin structure to facilitate replication initiation. All 6 genes encoding ORC (ORC1 through ORC6) may be required for initiation of DNA replication and are essential for viability. [ ]