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.