Finally, one of the best studied genes impacting on a wide variety of autoimmune diseases is the lymphoid tyrosine phosphatase (Lyp) gene, PTPN22. An R620W change is common in Europeans and increases the risk of type I diabetes, rheumatoid arthritis, systemic lupus erythematosus and several other autoimmune diseases [48]. The gene discussed is PTPN22; the disease is type 1 diabetes mellitus.