 The search for new specificities continues, both biochemically, by the analysis of cell-extracts, and computationally, by the analysis of sequenced genomes. Although most activities encountered today turn out to be duplicates-isoschizomers-of existing specificities, restriction enzymes with new specificities are found with regularity. Beginning in the early 1980’s, New England Biolabs embarked on a program to clone and overexpress the genes for restriction enzymes. Cloning improves enzyme purity by separating enzymes from contaminating activities present in the same cells. It also improves enzyme yields and greatly simplifies purification, and it provides the genes for sequencing and analysis, and the proteins for x-ray crystallography.  Restriction enzymes protect bacteria from infections by viruses, and it is generally accepted that this is their role in nature. They function as microbial immune systems. When a strain of E.coli lacking a restriction enzyme is infected with a virus, most virus particles can initiate a successful infection. When the same strain contains a restriction enzyme, however, the probability of successful infection plummets. The presence of additional enzymes has a multiplicative effect; a cell with four or five independent restriction enzymes could be virtually impregnable. The search for new specificities continues, both biochemically, by the analysis of cell-extracts, and computationally, by the analysis of sequenced genomes. Although most activities encountered today turn out to be duplicates-isoschizomers-of existing specificities, restriction enzymes with new specificities are found with regularity. Beginning in the early 1980’s, New England Biolabs embarked on a program to clone and overexpress the genes for restriction enzymes. Cloning improves enzyme purity by separating enzymes from contaminating activities present in the same cells. It also improves enzyme yields and greatly simplifies purification, and it provides the genes for sequencing and analysis, and the proteins for x-ray crystallography.  Restriction enzymes protect bacteria from infections by viruses, and it is generally accepted that this is their role in nature. They function as microbial immune systems. When a strain of E.coli lacking a restriction enzyme is infected with a virus, most virus particles can initiate a successful infection. When the same strain contains a restriction enzyme, however, the probability of successful infection plummets. The presence of additional enzymes has a multiplicative effect; a cell with four or five independent restriction enzymes could be virtually impregnable