CD40LG and rheumatoid arthritis: We also studied IgM-RF, which co-occurs with anti-CCP-2 (29 % in anti-CCP-2-negative and 91 % in anti-CCP-2-positive RA, P < 0.001), but again no increased frequency of IgM-RF was found in the subgroup of anti-CCP-2-negative multiplex-positive RA patients compared with multiplex-negative patients (25 % versus 33 %) [25].