At present, three different types of restriction-modification systems are known-types I, II, and III. Each system consists of two distinct enzyme activities: a DNA methylase and an endonuclease that catalyzes a double-strand DNA break. The type II sequence-specific endonucleases are the most widely used ones in molecular biology.
Properties of each of the three types of restriction systems are discussed in the links below and are summarized in Table 25.1:
INTERNET LINK: REBASE,
The Restriction Enzyme Database