In patients with MMN it may well be that anti-GM1 IgM antibodies are produced by a single or very few B-cell clones as shown by their restricted immunoglobulin light chain use (Cats et al., unpublished data) and the association of MMN with IgM monoclonal gammopathy (Vlam et al., unpublished data). This evidence concerns the gene CD40LG and monoclonal gammopathy.