An enzyme, specifically an endodeoxyribonuclease, that recognizes a short specific sequence within a deoxyribonucleic acid (DNA) molecule and then catalyzes double-strand cleavage of that molecule. Restriction enzymes have been found in bacteria, where they serve to protect against the deleterious effects of foreign DNA, and in viruses that infect Chlorella-like organisms, the only eukaryotes from which those enzymes have yet been isolated.