Members in the TNF superfamily regulate immune responses and induce apoptosis. A novel member in the TNF family was recently identified by several groups and designated BAFF (for B cell Activating Factor belonging to the TNF Family), BLyS (for B Lymphocyte Stimulator), TALL-1 (for TNF- and ApoL-related Leukocyte-expressed Ligand), and THANK (for TNF Homologue that Activate Apoptosis, NF-κ,B and c-jun N-terminal Kinase). BAFF/BLyS was characterized as a B cell activator since it induced B cell proliferation and immunoglobulin secretion. Three receptors for BAFF were recently identified and designated TACI, BCMA and BAFF-R. BAFF and its receptors are essential for B cell development, survival, and humoral immune responses. BAFF is involved in the development of autoimmune diseases including systemic lupus erythaematosus and rheumatoid arthritis.