HLA-DRB1 and rheumatoid arthritis: Using a metagenomic survey, Zhang et al. (28) reported that gut samples from RA patients were enriched in Gram-positive bacteria and depleted of Gram-negative bacteria; and demonstrated an existence of molecular mimicry between RA-associated antigens (e.g., collagen XI, HLA-DRB1) and microbial genes (e.g., from Clostridia) identified in gut and oral samples from RA patients.