CD95, also known as Fas and Apo-1, is a 40-50 kDa type I transmembrane glycoprotein and a member of the tumor necrosis factor receptor superfamily. It is expressed by activated lymphocytes, monocytes, neutrophils, fibroblasts and cell lines. Fas ligand binding to CD95 induces apoptosis in activated mature lymphocytes thereby playing a role in maintaining peripheral tolerance. Crosslinking of CD95 by the monoclonal antibodies DX2 and DX3 delivers an apoptotic signal to Fas-sensitive cells, indicating that these monoclonal antibodies recognize a functional epitope of CD95.1-5