Mutant Presenilin proteins cause early-onset familial Alzheimer's disease in humans and Caenorhabditis elegans Presenilins may facilitate Notch receptor signaling. We have isolated a Drosophila Presenilin homologue and determined the spatial and temporal distribution of the encoded protein as well as its localization relative to the fly Notch protein. In contrast to previous mRNA in situ studies, we find that Presenilin is widely expressed throughout oogenesis, embryogenesis, and imaginal development, and generally accumulates at comparable levels in neuronal and nonneuronal tissues. Double immunolabeling with Notch antibodies revealed that Presenilin and Notch are coexpressed in many tissues throughout Drosophila development and display partially overlapping subcellular localizations, supporting a possible functional link between Presenilin and Notch.