B lymphocyte stimulator (BLyS) is a recently identified member of the tumor necrosis factor (TNF) superfamily that costimulates B lymphocyte proliferation and differentiation. It prominently enhances the humoral responses to both T cell-independent and T cell-dependent antigens. BLyS is required to sustain proliferation of splenic germinal center (GC) B cell in vivo. As an antiapoptotic cytokine, BLyS provides survival signals by inducing up-regulation of Bcl-2. Transgenic mice overexpressing BLyS in lymphoid cells develops symptoms characteristic of systemic lupus erythaematosus (SLE).