Sural nerve biopsy from a case of IgM κ anti-MAG neuropathy shows moderate depletion of myelinated fibres in the resin section. Scattered thinly myelinated fibres are suggestive of remyelination. One such profile (arrow) shows poor staining of the myelin sheath indicating abnormal myelin periodicity (a). Sparse CD3+ endoneurial T cells are seen (b). Immunostaining for IgM strongly decorates the outer layers of the myelin sheaths of several axons (arrows) (c). Electron microscopy reveals the characteristic abnormal myelin period called widely spaced myelin resulting from increased separation of the major dense lines (arrows) (d) (Image courtesy of Zane Jaunmuktane and Sebastian Brandner)