Thiol protease (PubMed:30612035). Key protease responsible for the removal of the invariant chain from MHC class II molecules and MHC class II antigen presentation (PubMed:30612035). The bond- specificity of this proteinase is in part similar to the specificities of cathepsin L (PubMed:30612035). Elicits itch by mediating cleavage and activation of MRGPRX1 (By similarity). . This is the function of CTSS (cathepsin S, ENSG00000163131).