Families bearing mutations in the presenilin 1 (PS1) gene develop early onset familial Alzheimer's disease (FAD). Further, some PS1 mutants enhance secretion of the longer form of amyloid beta protein (Abeta42). We constructed cDNAs encoding human PS1 harboring 28 FAD-linked mutations, and examined the effects of the expressed PS1 mutants on Abeta42 secretion in beta amyloid precursor producing COS-1 cells. All the mutants significantly enhanced the ratio of Abeta42 to total Abeta compared with wild-type PS1. However, the increase in Abeta42 ratio in cells with each PS1 mutation did not correlate with the reported age of onset of FAD caused by that mutation. These results suggest that increased Abeta42 secretion is important for the development of Alzheimer's disease (AD), but may not be the only factor contributing to the onset of AD.