HLA-DRB1 and rheumatoid arthritis: 8 Genome-wide association studies have shown that PTN22, PADI4, STAT4, and TRAF1-C5 are associated with the onset of RA.9 Notably, the HLA-DRB1*0405 allele is closely associated with RA severity and susceptibility in Koreans.10,11 Clinical and experimental animal studies have shown that infection with Porphyromonas gingivalis, Proteus mirabilis, Epstein–Barr virus, or mycoplasma contributes to RA pathogenesis.12 The involvement of microbes in the etiopathogenesis of RA has prompted the investigation of relationships between RA and changes in human-associated microbial communities.