Kuru plaques are the pathologic hallmark in Gerstmann-Sträussler syndrome (GSS). To demonstrate that prion protein (PrP) is a component of kuru plaque cores, we fractionated and sequenced kuru plaque core derived peptides, following digestion with Achromobacter lyticus protease I. We identified 3 PrP-derived peptides by reverse-phase high-performance liquid chromatography and found a fragment of digests derived from a missense variant of PrP. The variant PrP was also present in the prion rod fraction in patients with GSS. This substitution may play a major role in cerebral amyloidogenesis.