Findings for β-diversity were heterogeneous, with some cohorts reporting significant associations with serostatus, whereas others found no clear separation.<h4>Conclusions</h4>Seropositive RA, especially ACPA-positive, is frequently associated with reduced microbial diversity and distinct compositional shifts compared with seronegative RA and healthy controls. The gene discussed is PRTN3; the disease is rheumatoid arthritis.