The prion protein (PrP) has been implicated both in prion diseases such as Creutzfeldt-Jakob disease, where its monomeric cellular isoform (PrPC) is recruited into pathogenic self-propagating polymers of misfolded protein, and in Alzheimer disease, where PrPC may act as a receptor for synaptotoxic oligomeric forms of amyloid-β (Aβ). This evidence concerns the gene PRNP and Alzheimer disease.