We performed a study of the distribution of PrP27-30, the proteinase-K-resistant form of prion protein, in the central and peripheral nervous system of a patient with a Glu200Lys mutation of the prion protein gene, cerebellar ataxia, subcortical dementia, rigidity, and demyelinating peripheral neuropathy. In the CNS, there was neuron loss and spongy degeneration, principally in the cerebellum, and with progressively lower density in the caudate nucleus, thalamus, temporal cortex, frontal cortex, and brainstem. Evaluation of the expression of PrP27-30 by Western blot showed that its distribution correlated with the intensity of the lesions in these regions. In contrast, we did not detect PrP27-30 in the peripheral nervous system where lesions consisted of demyelination, and remyelination that predominated in the proximal nerve trunks and roots.