How are restriction endonuclease enzymes named ?

Naming of Restriction enzymes is  done according to the convention which as follows :

1. The first letter of the name is taken  from the genus.

2.The next two letters comes from the name of the species of the cell from which they are isolated.

3. The next letter comes from the strain of the organism.

4. The roman number following these four letters indicate the order in which the enzymes was isolated from that strain of the bacterium.

Example :EcoR I is isolated from Escherichia coli. R Y 13 strain , and it was the first restriction enzyme isolated from E.coli.