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Presenilin (PS) facilitates gamma-secretase cleavage of the beta-amyloid precursor protein and the intramembraneous cleavage of Notch1 (1). Presenilin (PS) is essential for the gamma-cleavage required for the generation of the C terminus of amyloid beta-protein (Abeta). The discovery that a deficiency of presenilin 1 (PS1) decreases the production of amyloid beta-protein (Abeta) identified the presenilins as important mediators of the gamma-secretase cleavage of beta-amyloid precursor protein (APP). It has been shown that two conserved transmembrane (TM) aspartates in PS1 are critical for Abeta production, providing evidence that PS either functions as a required diaspartyl cofactor for gamma-secretase or is itself gamma-secretase. Presenilin 2 (PS2) shares substantial sequence and possibly functional homology with PS1. Here, we show that the two TM aspartates in PS2 are also critical for gamma-secretase activity, providing further evidence that PS2 is functionally homologous to PS1 (2). PS are found primarily within intracellular membranes, including the endoplasmic reticulum (ER) and the trans-Golgi network as well as the plasma membrane. PS are also expressed in most cell types throughout development (3).