Intensive studies of three proteins--Presenilin, Notch, and the amyloid precursor protein (APP)--have led to the recognition of a direct intersection between early development and late-life neurodegeneration. Notch signaling mediates many different intercellular communication events that are essential for determining the fates of neural and nonneural cells during development and in the adult. The Notch receptor acts in a core pathway as a membrane-bound transcription factor that is released to the nucleus by a two-step cleavage mechanism called regulated intramembrane proteolysis (RIP). The second cleavage is effected by Presenilin, an unusual polytopic aspartyl protease that apparently cleaves Notch and numerous other single-transmembrane substrates within the lipid bilayer. Another Presenilin substrate, APP, releases the amyloid ss-protein that can accumulate over time in limbic and association cortices and help initiate Alzheimer's disease. Elucidating the detailed mechanism of Presenilin processing of membrane proteins is important for understanding diverse signal transduction pathways and potentially for treating and preventing Alzheimer's disease.