The aim of this study was to investigate different RF isotypes (IgM RF, IgA RF, and IgG RF) in patients with SS alone, SS complicated with polyarthritis (pa) as an EGM (SS+pa), and SS associated with rheumatoid arthritis as a second autoimmune disorder (SS+RA) to find out whether differences between RF isotype patterns of the groups could be potentially useful in everyday practice. The gene discussed is CD40LG; the disease is synovial sarcoma.