PTPN22 and rheumatoid arthritis: The list of single nucleotide polymorphisms (SNPs) associated with RA is long and includes numerous non-HLA genes, including TRAF1, STAT4, CTLA4, IRF5, CCR6, PTPN22, IL23R, PADI4, GPC5, FCRL, RBFOX1, IL6R, SPRED2, PXK, RBP1, TAGAP, TAGAP, CCL3, CARD8, CD40, IRAK1, GATSL3, and MTHFR [1,3,10,13,14].