CD40LG and rheumatoid arthritis: To elucidate this observation further, we investigated the correlation between the IgM reactivity against EBNA-1 and the level of rheumatoid factors of IgM isotype, which indeed revealed a strong correlation (Figure S4). Thus, the difference between RA patients and controls regarding the IgM reactivity against EBNA-1 is presumably caused as a pitfall by the presence of rheumatoid factors of IgM isotype in RA patient sera.