Restriction endonucleases bind to specific sequences of bases in duplex DNA and cleave the DNA once or twice for each occurrence of that sequence.These enzymes are remarkable for their accuracy and for their great variety:hundreds have been discovered,each differing in the particular sequence bound and the exact position of the cleavage.From the restriction enzymes point of view,DNA represents a double substrate whose cleavage requires the action of two catalytic sites,one for hydrolyzing each strand.Several strategies have evolved for incorporating two sites into one enzyme.