Simultaneously four different laboratories identified a new member of the tumor necrosis factor (TNF) family. This has been named as TALL-1 , THANK (TNF homologue that activates apoptosis, nuclear factor-kappaB, and c-Jun NH2-terminal kinase , BAFF (for B cell activating factor belonging to the TNF family) and Blys (B lymphocyte stimulator). Membrane-bound BAFF is processed and secreted through the action of a protease whose specificity matches that of the furin family of proprotein convertases. Secreted BLyS/BAFF/THANK acts as a potent B cell growth factor. Overexpression of BLyS/BAFF in transgenic mice lead to increased numbers of mature B and effector T cells. These mice also develop autoimmune like symptoms, such as, high levels of rheumatoid factors, anti-DNA autoantibodies, etc.. Recently, two receptors for BLyS/BAFF have been identified and termed as BCMA and TACI. 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. BAFF and its receptors are essential for B cell development, survival, and humoral immune responses.