Serine proteases of MEROPS family S1, subfamily A, are endopeptidases that cleave peptide bonds in which serine serves as the nucleophilic amino acid at the active site. These enzymes are generally transported to extracellular, vesicular, or granular locations, where their functions include digestion and defense responses. Serine proteases are commonly synthesized as zymogens - these can form a cascade pathway in which one protease activates the zymogen of another to mediate a rapid, local reaction. (Adapted from FBrf0155784 & FBrf0240739.)