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 Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker's syndrome, and Kuru. A mutation in the open reading frame of this gene was recently described in a cluster of patients with CJD in Slovakia. This mutation at codon 200 changes glutamic acid coded by GAG to lysine coded by AAG. We examined the prevalence of this mutation in the cluster of patients with CJD among Sephardic Jews of Libyan descent in Israel. A polymerase chain reaction was used to amplify the open reading frame of the prion protein gene from DNA extracted from frozen brain tissue of five Israeli residents (four of Libyan and one of Greek origin) and two familial cases in Jews born in Greece and Tunisia who later emigrated to France. The existence of the codon 200 mutation was detected by digestion of the open reading frame fragments with the BsmA1 restriction enzyme. All patients had the same codon 200 mutation. These findings implicate this mutation in the high prevalence of CJD among Libyan and Sephardic Jews from other Mediterranean countries.