Rheumatoid arthritis and SLE share several risk-associated loci (e.g., HLA-DRB1, BLK, UBE2L3, PTPN22, STAT4, TNFAIP3, FCGR2A, PRDM1, IRF5, PXK and COG6) and are characterized by the presence of autoantibodies that recognize self-antigens (15, 16, 31, 94). Here, PTPN22 is linked to systemic lupus erythematosus.