Restriction enzymes are exceedingly varied; they range in size from the diminutive PvuII (157 amino acids) (NEB #R0151) to the gigantic CjeI (1250 amino acids). Among the over 3,000 activities that have been purified and characterized, more than 250 different sequence-specificities have been discovered. Of these, over 30% were discovered and characterized at New England Biolabs. 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 regularly.
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, while providing the genes for sequencing and analysis, and the proteins for X-ray crystallography.