Programmed cell death (apoptosis) may be induced in response to a variety of cytotoxic stimuli, including activation of surface receptors. An area of particular interest has been the induction of apoptosis by the receptor-ligand pair: Fas (CD95) and Fas Ligand (FasL). Fas is an ~45 kD cell surface protein which belongs to the the TNF (tumor necrosis factor receptor family, and is expressed in various tissue and cells including the thymus, liver, ovary and lung. FasL, a member of the TNF ligand family, is expressed on activated T and NK cells. FasL initiates signaling at the cell surface by aggregation of individual Fas recpetors via binding to the multivalent ligand. Antibodies which selectively activate Fas can be used in vitro to mimic the apoptotic response associated with FasL. Both Fas and FasL are thought to play an important role in the apoptotic processes that take place during T cell development. Clone G254-274 recognizes human Fas. A soluble form of recombinant human Fas (lacking the Fas transmembrane domain) was used as immunogen.