In the- late 1960's, scientists Stewart Linn and Werner Arber isolated samples of the two types of enzymes responsible for phage growth restriction in Escherichia coli (E coli) bacteria. One of these enzymes methylated DNA, while the other cleaved unmethylated DNA at a wide variety of locations along the length of the molecule. The first type of enzyme was called a "methylase" while the other was called a "restriction nuclease."