Three members of a Basque family carrying a novel six R2 octapeptide repeat 144-bp insertion in the prion protein gene (PRNP) showed a slowly progressive dementia associated with cerebellar signs, myoclonic jerks, and seizures. Although postmortem examination revealed only focal and minimal spongiform degeneration in one subject with a 4-year course, significant astrogliosis and neuronal loss were associated with pronounced spongiform degeneration in the patient with a duration of symptoms of 10 years. Prion protein (PrP)-immunoreactive patches with a unique morphology were present in the molecular layer of the cerebellum in both subjects. Western blot analysis demonstrated the presence of protease-resistant prion protein (PrP(res)) with the same characteristics (size and ratio of the three differently glycosylated isoforms) of that found in typical sporadic Creutzfeldt-Jakob disease (CJD129M/M, PrP(res) type 1). The amount of PrP(res) correlated with presence and severity of spongiform degeneration in the cerebral cortex. The findings suggest that a relatively low rate of PrP(res) deposition is the cause of the lack of spongiform degeneration in subjects carrying a 144-bp insertion in PRNP. The presence of PrP-immunoreactive patches with unique morphology in the molecular layer of the cerebellum is a hallmark of certain prion encephalopathies with insertional mutations and is useful in the diagnosis of this subtype of human prion disease.