FAS receptor (CD95, APO-1) is a transmembrane TNF death receptor that induces apoptosis via FAS ligand binding and is centrally involved in the regulation of programmed cell death. FAS receptor and its ligand FASLG are implicated in the pathogenesis of a number of cancers and immune diseases such as multiple sclerosis. FAS also plays a role in apoptotic cell loss in neurodegenerative diseases. Furthermore, it is upregulated and participates in phenylalanine-induced cell death in cortical neurons in patients with phenylalanine hydroxylase mutations, leading to childhood mental retardation. In immunohistochemistry, FAS has membranous and also secretory positivity at variable levels in most tissues throughout the body. References: Cell Death and Differentiation. 11 Suppl 1 (Suppl 1): S23–7, PMID: 15143352; Nature. 465 (7297): 492–6, PMID: 20505730; PLoS One. 2013 Aug 7;8(8):e71553, PMID: 23940767; Lab Invest. 1998 Apr;78(4):401-11, PMID: 9564885;