Taking into account that cyclosporine is frequently used in the treatment of various autoimmune diseases, such as psoriasis, rheumatoid arthritis, myasthenia gravis, systemic lupus erythematosus and diabetes mellitus, the identification of MDR1 polymorphisms may help us to find the cause of the ineffectiveness of their therapy. The gene discussed is ABCB1; the disease is myasthenia gravis.