The cluster of Creutzfeldt-Jakob disease (CJD) among Jews of Libyan origin is one of the largest in the world. A number of hypotheses have been proposed to account for this cluster, the most prevalent but unsubstantiated hypothesis being that a transmissible agent was ingested in the form of scrapie-infected sheep brains. It has, however, been shown that a modified host protein encoded by the gene specifying the scrapie amyloid precursor is critically involved in the pathogenesis of transmissible spongiform encephalopathies such as CJD, Gerstmann-Sträussler-Scheinker syndrome and Kuru. A mutation at codon 200 in the open reading frame of this gene has recently been linked to a cluster of CJD patients in Slovakia. We examined the prevalence of this mutation among CJD patients of Libyan descent in Israel. All patients were found to have the same codon 200 mutation. These findings implicate this mutation in the high prevalence of CJD among Libyan Jews and Sephardic Jews from other Mediterranean countries