1967 - The serine proteases are a historically very important group of enzyme structures, because collectively they illuminated catalytic mechanism (in their case, by the Ser-His-Asp "catalytic triad"), the basis of differing substrate specificities, and the activation mechanism by which a controlled enzymatic cleavage buries the new chain end to properly rearrange the active site.[16] The early crystal structures included chymotrypsin (PDB file 2CHA),[17] chymotrypsinogen (PDB file 1CHG),[18] trypsin (PDB file 1PTN),[19] and elastase (PDB file 1EST).[20] They also were the first protein structures that showed two near-identical domains, presumably related by gene duplication. One reason for their wide use as textbook and classroom examples was the insertion-code numbering system (hated by all computer programmers), which made Ser195 and His57 consistent and memorable despite the protein-specific sequence differences.