CD178.1 is a 40 kD Fas ligand alloantigen (designated as mFasL.1) expressed on the activated T lymphocytes, testis, and eye of selected mouse strains including C57BL/6, C3H, MRL, NOD, NZB, NZW, and SJL. It does not react with lymphoblasts from BALB/c, DBA/1, or DBA/2 mice. CD178.1 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. Functional studies suggest that mFasL.2 has higher specific activity than mFasL.1. 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 been implicated in the proliferation of CD8+ cells and neutrophil extravasation, chemotaxis, and survival. The Kay-10 antibody has been reported to block CD178.1/CD95 induced apoptosis in cells derived from mice expressing this alloantigen.