An impaired CD40-CD154 interaction leads to humoral and cellular immunodeficiency; thus, the CD40-CD154 co-stimulatory pathway is associated with the pathogenesis of several diseases, including autoimmune thyroiditis, type 1 diabetes, inflammatory bowel disease, psoriasis, multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus (4,5). The gene discussed is CD40; the disease is rheumatoid arthritis.