CTSS (Cathepsin S) is a lysosomal cysteine protease of the papain family that functions as a potent elastase in alveolar macrophages and degrades proteins for antigen presentation to the MHC Class II complex. It is found in macrophages, B-lymphocytes, microglia, dendritic cells and populations of epithelial cells. Due to its ability to cleave a wide range of substrates and its role in blood vessel angiogenesis and permeability, it also promotes tumor growth. It is a prognostic marker for a number of cancers including type IV astrocytomas, where it is upregulated, and also a target of inhibition therapies. Suppression of cathpesin S has also been shown to be protective after brain injury. In immunohistochemistry of normal tissue, CTSS has cytoplasmic positivity throughout most of the body. References: Am J Pathol. 2003 Jul;163(1):175-82, PMID: 12819022; Mediators of Inflammation. 2013 (2013): 187873, PMID: 24282339; Mol Cancer. 2014 Mar 2;13:43, PMID: 24580730; Mol Cancer. 2016 Apr 21;15:29, PMID: 27097645; Neoplasma. 2015;62(1):16-26, PMID: 25563363; Cancer Genomics Proteomics. 2009 May-Jun;6(3):149-59, PMID: 19487544