PTPN22 and systemic lupus erythematosus: The PTPN22 gene, encoding the lymphoid tyrosine phosphatase LYP, has emerged as one of the major non-HLA risk factors for a wide range of autoimmune diseases, including type 1 diabetes, rheumatoid arthritis (RA), systemic lupus erythematosus, Graves’ disease and others [1], [2].