Cathepsin S is also known as CTSS, which is a member of the peptidase C1 family. Cathepsin S is a lysosomal cysteine protease that may participate in the degradation of antigenic proteins to peptides for presentation on MHC class II molecules. Cathepsin S can function as an elastase over a broad pH range in alveolar macrophages. The catalytic activity of cathepsin S is similar to cathepsin L, but with much less activity on Z-Phe-Arg-|-NHMec, and more activity on the Z-Val-Val-Arg-|-Xaa compound.