It is a truism of modern biomedical science that the development of therapies expected to slow or arrest the progression of a disease requires as detailed an understanding of its molecular and cellular pathogenesis as possible. In turn, the cloning of novel gene products implicated in a disease often leads to new insights about fundamental features of protein structure and function. A particularly compelling example of this beneficial interplay between basic and applied cell biology arises from the exciting recent progress in deciphering Alzheimer's disease (AD). This review discusses the current understanding of the cell biology of two proteins crucial for the pathogenesis of AD, the beta-amyloid precursor protein and presenilin.