MRL-lpr/lpr mice bear mutations in the gene encoding Fas and serve as a widely used model for autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren’s syndrome (SS), and autoimmune lymphoproliferation syndrome (ALPS) (10, 11). The gene discussed is FAS; the disease is rheumatoid arthritis.