IgM monoclonal gammopathy has been reported in some patients with motor neuron disease. The monoclonal IgMs in several of the patients bind to the carbohydrate epitope Gal (beta 1-3) GalNAc, which is shared by gangliosides GM1 and GD1b and glycoproteins in the nervous system and crossreacted with Gal (beta 1-3) GlcNAc. They also immunostain spinal cord and gray matter and presynaptic terminals of motor neurons at the neuromuscular junction. The role and mechanisms of action of these antibodies in motor neuron disease is under investigation.