Alzheimer's disease, which is characterized by progressive loss of memory and cognitive function, affects 15 million people worldwide. The incidence increases steadily from 0.5 percent per year at the age of 65 years to nearly 8 percent per year after the age of 85 years.¹ Because survival for a decade is common, the prevalence increases from 3 percent at the age of 65 years to 47 percent after the age of 85 years.² Mutations in the gene for the amyloid precursor protein and the genes for presenilin 1 and 2 cause rare, dominantly inherited forms of the disease occurring before . . .