The altered expression of PrPC in aging and the development of AD are associated with disease progression, and it has been observed that PrPC is decreased in the hippocampus and temporal cortex in aging and sporadic AD but not in familial AD, suggesting that PrPC expression reduced reflects a main mechanism of disease and is not merely a minor consequence of other AD-associated changes (Whitehouse et al., 2010). This evidence concerns the gene PRNP and Alzheimer disease.