A change of the prion protein conformation results in a class of neurodegenerative diseases called the transmissible spongiform encephalopathies (like mad cow and Creutzfeld-Jakob diseases). The function of the normal prion protein is unknown, although much of recent research demonstrates the it may be a copper binding protein selective for Cu(II). Amyloid precursor protein (APP) releases the 39-42 amino acid peptide, a major constituent of the deposit in plaques of Alzheimer disease brain. Also APP is a metal binding protein, including copper ions. The link between copper and both proteins may provide insight into the role of metals in neurodegenerative pathologies.