CD178 is a 40 kD member of the TNF/NGF superfamily also known as Fas ligand, FasL, Apo-1 ligand, and CD95 ligand. Cell surface CD178 is expressed on activated T cells and in testis and eye. CD178 is upregulated in activated T cells upon TCR re-engagement and has been shown to induce autocrine and paracrine T cell death. CD178 expression in the eye and testis has been shown to participate in immune privilege at these sites. CD178 can be cleaved from the surface by metalloproteases, and "soluble" CD178 may block the activities of membrane-bound CD178. CD178 binds to CD95 (Fas) to induce apoptotic cell death implicated in the maintenance of peripheral tolerance. CD178/CD95 interactions have also been implicated in the proliferation of CD8+ cells and neutrophil extravasation, chemotaxis, and survival. The MFL4 antibody recognizes CD178 in a wide array of mouse strains and has been reported to block CD178/CD95 induced apoptosis. The MFL4 antibody is cross-reactive with rat and mouse FasL.