A protein complex that functions as an endonuclease to cleave DNA at or near a specific recognition site, when that site is unmethylated. These complexes may be dimers or tetramers; it is also possible for the endonuclease to be in a complex with the corresponding methyltransferase that methylates the recognition site. DNA restriction systems such as this are used by bacteria to defend against phage and other foreign DNA that may enter a cell.