FAS / CD95 is a 36kDa transmembrane type I receptor， which belongs to the NF/NGF receptor super-family. It is a potent inducer of apoptosis in cells of the immune system upon interaction with its natural ligand， CD95L. Although the CD95/CD95L system could play a role in tumor regression， tumor cells seem to down-regulate CD95 expression as a mechanism of resistance to CD95L-induced killing by T lymphocytes and NK cells. The Fas antigen is expressed on the surface of various cell types， including activated T and B lymphocytes and T lymphoblastoid cell line. CD95 is also expressed in a broad panel of non-transformed cells outside the immune system.
