Peptidase family S1 comprises endopeptidases in which serine serves as the nucleophilic amino acid at the active site. It is the largest protease family and includes both catalytically active and inactive enzymes, termed 'serine proteases' and 'serine protease homologs, respectively. These enzymes play essential roles in a wide range of physiological processes including digestion, defense and development. (Adapted from FBrf0239469 and FBrf0155784.)