PTPN22 and rheumatoid arthritis: The single nucleotide polymorphism (SNP) rs2476601 (+1858C/T), located in exon 14 of the PTPN22 gene, has previously been found associated with several autoimmune diseases, for example, diabetes type-I [11] and RA, with a stronger association with ACPA sero-positive RA [12,13].