The presenilins (PS-1 and PS-2) are 2 members of a novel family of genes encoding integral membrane proteins recently implicated in Alzheimer's disease (AD) pathology. To date, 43 mutations have been identified in PS-1 and 2 in PS-2 that lead to familial presenile AD (onset before age 65 years). The normal and pathological functions of the PS proteins (ps-1 and ps-2) are unknown, but their high degree of homology predicts similar biological activities. Homologies with ps from other species suggest that they may play a role in intracellular protein sorting and trafficking, in intercellular cell signaling, or in cell death. Since to date only missense mutations and in-frame deletions were identified, it is believed that mutated ps act through either a gain of (dys-)function or a dominant negative effect. In vivo and in vitro studies have linked PS mutations to amyloid deposition, an early pathological event in AD brains.