Serine proteases, named for the nucleophilic Ser residue at the active site, are a large and diverse group of proteases (accounting for approximately one-third of all proteases) that are characterized by the presence of the Asp-His-Ser “charge relay” system (catalytic triad) in their chemical structures. However, with the development of technology, a variety of novel serine proteases with catalytic triads and dyads have been discovered, including Ser-His-Glu, Ser-Lys/His, His-Ser-His and N-terminal Ser. Serine proteases are traditionally divided into four clans based on the Asp-His-Ser triad in different structural contexts, including chymotrypsin-like proteases, subtilisin-like proteases, carboxypeptidase Y-like proteases and Clp protease-like proteases.