In the year $1963$, two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One of these added methyl groups to $DNA$. while the other cut $DNA.$

The enzymes which cut $DNA$ are called, restriction endonuclease.

Hind-$II$ is a first restriction endonuclease enzyme.

Hind-$II$ always cut $DNA$ molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind-$II.$

Besides Hind-$II$, today we know more than 900 restriction enzymes that have been isolated from over $230$ strains of bacteria.

Nomenclature – Naming : The naming of these enzymes is done from which bacteria they are obtained on that basis, it is done by either three or four letters.

E.g., $EcoRI$ comes from Escherichia coli, the letter ' $\mathrm{R}^{\prime}$ is derived from the name of strain Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria.