Serum levels of BAFF and APRIL increase in some autoimmune diseases, such as systemic lupus erythematosus (SLE), Sjögren’s syndrome (SjS), rheumatoid arthritis, immunoglobulin G4 (IgG4)-related disease, and anti-Jo-1-positive polymyositis and dermatomyositis [14–18]. The gene discussed is TNFSF13; the disease is autoimmune disease.