A heterodimeric enzyme complex composed of two subunits, Res and Mod, that functions as an endonuclease and cleaves DNA. Cleavage will only occur when there are two un-methylated copies of a specific recognition site in an inverse orientation on the DNA. Cleavage occurs at a specific distance away from one of the recognition sites. The Mod subunit can act alone as a methyltansferase. DNA restriction systems such as this are used by bacteria to defend against phage and other foreign DNA that may enter a cell.