Cathepsin S is a protein that in humans is encoded by the CTSS gene. Transcript variants utilizing alternative polyadenylation signals exist for this gene. Cathepsin S is a member of the peptidase C1 family of cysteine cathepsins, a lysosomal cysteine protease that may participate in the degradation of antigenic proteins to peptides for presentation to the MHC class II. Cathepsin S can function as an elastase over a broad pH range in alveolar macrophages. Cathepsin S is a lysosomal enzyme that belongs to the papain-like protease of cysteine proteases. While a role in antigen presentation has long been recognized, it is now understood that cathepsin S has a role in itch and pain, or nociception . The nociceptive activity results from cathepsin S functioning as a signaling molecule via activation of protease-activated receptors 2 and 4 members of the G-protein coupled receptor family.