PTPN22 and rheumatoid arthritis: The variants may differ in their associated RA susceptibility, age of onset, sex bias, serum factors, or synovial fluid factors (e.g., rs1800896 [–1082G/A], PTPN22 rs2476601, IL8 rs2227306 [781C/T], IL6 rs1800795 [–174G/C], IL2 [–330G/T], and TNFA rs1800629 [–308A/G]) [17,18,19].