Amyloid β: Walking on the dark side of the moon.

For some decades, amyloid β (Aβ) has only been considered as a cytotoxic peptide, putative cause and marker of Alzheimer's disease (AD). Today, however, a considerable amount of evidence goes against the classical amyloid hypothesis and illustrates a new picture in which the Aβ loss of function, rather than its accumulation, has a pathogenic role in AD. In this concise review, we summarize some highlights of a collection of research pointing to the physiological function of Aβ and its role in the mechanisms of memory formation.