The Obama administration has announced a bold research program to test whether a drug can prevent the onset of Alzheimer’s disease well before any symptoms appear. It is a long shot, but the payoff could be huge.

Currently, there is no cure for Alzheimer’s, which steadily robs patients of their memory, followed by full-blown dementia. There is also no diagnostic test to identify who has it, and no treatment to slow patients’ deterioration for more than a few months.

While work continues on those fronts, the new clinical trial will test whether the drug, Crenezumab, made by Genentech, can prevent the disease in a group of people whose genetic heritage guarantees that they will develop it. If the drug successfully prevents the loss of mental capacities as measured by a sensitive new cognitive test there is hope — but no guarantee — that it could do the same for members of the general public. As Pam Belluck described in The Times last week, the trial will focus on members of an extended family in Colombia who carry a rare genetic mutation that causes them to develop Alzheimer’s early in life. They typically experience cognitive impairment at about age 45 and dementia by 51. The trial will also include a smaller number of individuals in the United States with the same genetic mutation.

Instead of recruiting thousands of volunteers and following them for an extended period as in a customary prevention trial, the researchers in Colombia will give the drug to only 100 people with the early-onset genetic mutation. They will give placebos to another 100 people with the mutation and to 100 family members who do not carry the deadly gene.