Several alleles in the epitope-recognition part of the HLA molecule that show the highest association with RA susceptibility, also share a common string of amminoacid residues (the so-called shared-epitope hypothesis). Other variants in potentially pathogenic genes located in non-MHC regions have been implicated by recently performed genome wide analysis studies. These genes include PTPN22, TRAF1-C5, PADI4, STAT4.