Precedent of causative multiplication of key gene loci exists in familial forms of both Alzheimer's and Parkinson's diseases. Genetic Creutzfeldt-Jakob disease (CJD) is often clinically indistinguishable from sporadic disease and inexplicably, a negative family history of a similar disorder occurs in around 50-90% of patients harboring the most common, disease-associated, prion protein gene (PRNP) mutations. We undertook semi-quantitative analysis of the PRNP copy number in 112 CJD patients using quantitative polymerase chain reaction. All included cases satisfied classification criteria for probable or definite sporadic CJD, ascertained as part of longstanding, prospective, national surveillance activities. No examples of additional copies of the PRNP locus as an explanation for their disease was found in any of the 112 sporadic CJD patients. Hence, contrasting with more common, age-related neurodegenerative diseases, the genetic aetiology in human prion disease continues to appear entirely restricted to small scale mutations within a single gene, with no evidence of multiplication of this validated candidate gene locus as a cause.