Patients with autoimmune diseases such as SLE often have increased blood levels of B lymphocyte stimulator (BLyS), also known as B cell activating factor [5–7], a member of the tumour necrosis factor ligand superfamily that promotes the survival of B lymphocytes, inhibits B cell apoptosis and is involved in the differentiation of B cells into immunoglobulin-producing plasma cells [8–11]. This evidence concerns the gene TNFSF13B and systemic lupus erythematosus.