Systematic name and number which identify an enzyme in technical literature. Assigned by the Enzyme Commission, the EC Number consists of four numbers separated by dots: the first classifies the enzyme into one of the six broad groups: 1. Oxidoreductase; 2. Transferases; 3. Hydrolases; 4. Lyases; 5. Isomerases; 6. Ligases. Each group is subdivided into sub-groups, each sub-group into sub-sub-groups, and the last number is specific for the enzyme, e.g., EC 3.1.21.1 is deoxyribonuclease I.