PTPN22 has been identified as the main susceptible gene for multiple autoimmune diseases, including rheumatoid arthritis [23], juvenile idiopathic arthritis [24], psoriatic arthritis [25], systemic lupus erythematosus [26], systemic sclerosis [27] and some forms of vasculitis [28]. This evidence concerns the gene PTPN22 and rheumatoid arthritis.