The C1858T polymorphism within the protein tyrosine phosphatase PTPN22 (encoding PTPN22R620W) is a major risk factor for the development of multiple autoimmune diseases, including rheumatoid arthritis (RA), type I diabetes, lupus and juvenile idiopathic arthritis (JIA) [1]. Here, PTPN22 is linked to rheumatoid arthritis.