The DX2 monoclonal antibody specifically binds to the human Fas antigen (also called APO-1). This 45 kDa type I transmembrane glycoprotein was designated as CD95 at the Fifth HLDA Workshop. Fas is a member of the TNF-receptor superfamily and is also known as Tumor necrosis factor receptor superfamily member 6 (TNFRSF6). It is differentially expressed on a variety of normal and neoplastic cells. These include some undifferentiated thymocytes, and activated T and B lymphocytes, natural killer (NK) cells, monocytes, neutrophils, fibroblasts, and cell lines. CD95 is preferentially expressed on CD45RO-positive memory T lymphocytes and γ/δ T lymphocytes. The Fas/CD95 antigen is a polypeptide that plays a role in the programmed sequence of events leading to cell death, termed apoptosis. Crosslinking CD95 with DX2 antibody delivers an apoptotic signal indicating that DX2 recognizes a functional epitope of the CD95 antigen.