In some cases of PN, the M (monoclonal) protein plays a definite pathogenic role, for example, the neuropathy associated with immunoglobulin M (IgM) anti–myelin-associated glycoprotein (anti-MAG) and in patients with Waldenström macroglobulinemia and IgM monoclonal gammopathy of undetermined significance (Gawler, 2004). The gene discussed is CD40LG; the disease is Waldenstrom macroglobulinemia.