A preventive treatment for Alzheimer’s disease is one of medicine’s holy grails. Until recently, however, testing such a regimen would have been impossible—people do not have symptoms of dementia until it is too late. Now the Dominantly Inherited Alzhei­mer Network project, a large international study of those whose families suffer from a heritable form of early-onset Alzheimer’s, has found that those who develop the disease have chemical changes in their brain decades before symptoms appear. Although the genetic form of the disease is rare, the discovery of these early chemical signals gives scientists a much needed group of people they can use to test potential deterrents. If a drug works on them, it would probably help the rest of the population, explained scientists at the International Conference on Alzheimer’s in Paris this past July. [For more on these families and the quest to prevent Alzheimer’s, see “Decoding Dementia,” by Joel Shurkin; Scientific American Mind, November/December 2009.]