As we demonstrate here that APA catalyzes the removal of the first aspartyl residue of Aβ, and thus trigger the first catalytic step of N-terminal truncation subsequently leading to pE3-42Aβ, we postulated that AD-related accumulation of pE3-42Aβ could be accounted for by an increased activity of APA. The gene discussed is ENPEP; the disease is Alzheimer disease.