PRNP and early-onset autosomal dominant Alzheimer disease: By this definition, fewer than 25 amyloid-forming proteins have been identified and associated with serious diseases, including amyloid-β peptide (Aβ) with Alzheimer's disease (AD), islet amyloid polypeptide (IAPP) with diabetes type 2, and prion protein (PrP) with the spongiform encephalopathies [3].