PTPN22 and systemic lupus erythematosus: Investigations using a candidate gene approach and genome-wide association studies have demonstrated that RA, SLE, and GD susceptibility is conferred by different variants of protein-coding genes, including HLA-class II, PTPN22, TNFAIP3, STAT4, TNFRSF14, BLK, and TRAF1 [8–10].