HLA-DRB1 and rheumatoid arthritis: Prevotella_7 was associated with HLA-DRB1 shared-epitope risk alleles for rheumatoid arthritis in the SCREEN-RA cohort (n=133; q=0·035); the association was stronger in a subgroup analysis in which 44 participants with swollen joints were removed to isolate genotype from rheumatoid arthritis pathophysiology (n=89; q=0·0011; figure 4).