The trial is not expected to begin until 2012 because researchers are applying for federal financing and have not yet decided which drug to test. Testing will occur in the region where most relatives live, Antioquia, which includes Medellin and many isolated mountain villages.

But last month, 12 relatives visited Phoenix so scientists could conduct PET scans that can show whether their brains have the characteristic amyloid plaques of Alzheimer’s disease. Altogether, these scans will be performed on 50 family members this fall, some with Alzheimer’s already, some with the mutation that will cause it, and some who have no mutation and will not get the disease.

The snapshots of amyloid in family members with and without the gene, and with and without symptoms, will help focus the drug-testing study so researchers can better understand whether the drug is staving off Alzheimer’s, said Dr. Eric Reiman, the Banner Institute’s executive director.

“We need to find out when these amyloid plaques accumulate, how advanced they are by the time they enter the prevention trial,” Dr. Reiman said. “This information will provide a foundation for knowing how much these brain changes have occurred roughly at the time people at their age will enroll in the trial.”

The drug trial will test a treatment that attacks amyloid, most likely a drug already tried unsuccessfully in people with Alzheimer’s symptoms. Many scientists now believe it is possible that drugs have failed so far because once symptoms begin, the brain is already badly damaged.