These are altered restriction enzymes that hydrolyze only one strand of the duplex, to produce DNA molecules that are "nicked", rather than cleaved. These conventional nicks (3´-hydroxyl, 5´-phosphate) can serve as initiation points for a variety of further enzymatic reactions such as replacement DNA synthesis, strand-displacement amplification.
Nicking endonucleases are as simple to use as restriction endonucleases. Since the nicks do not fragment DNA, their activities are monitored by conversion of supercoiled plasmids to open circles by nicking; alternatively, substrates with nicking sites close enough on opposite strands to create a double-stranded cut can be used instead