Prion diseases are now a focus of attention since the diseases are known to disseminate beyond expectation. They are various types of Gerstmann-Sträussler-Scheinker syndrome(GSS), fatal familial insomnia, iatrogenic Creutzfeldt-Jakob disease(CJD), and variant CJD which suspected to have disseminated from bovine spongiform encephalopathy. Abnormal prion protein deposits as amyloid structure in the brain of the patients with these diseases. Mutation of the prion protein gene exists in a part of these diseases. Depending on their kind, patients represent a variety of clinical and neuropathological manifestations. They can be clinically divided into two general groups of the CJD type and the GSS type. For clinical diagnosis of the GSS type prion diseases, analysis of prion protein gene is useful.