The enzyme classification and nomenclature list was first approved by the International Union of Biochemistry in 1961. Six enzyme classes have been recognized based on the type of chemical reaction catalyzed, including oxidoreductases (EC 1), transferases (EC 2), hydrolases (EC 3), lyases (EC 4), isomerases (EC 5) and ligases (EC 6). However, it was not noticed back at the time that none of these classes clearly describe an important group of enzymes, which move ions or molecules across membranes or their separation within membranes. Although the hydrolytic reaction is not the primary function, several of these enzymes are involved the hydrolysis of ATP had been previously classified as ATPases (EC 3.6.3.-). Nowadays, these enzymes have been classified under a new EC class of translocases (EC 7).