More than 6 million Americans, and millions more worldwide, have Alzheimer’s. A handful of genes are known to cause rare “early-onset” forms, mutations passed through families that trigger symptoms unusually young, by age 50. Some cases also are linked to Down syndrome.

In a new paper in Nature Medicine, an international team of neurologists makes the compelling case that people with two copies of a gene called APOE4 aren’t just at risk of Alzheimer’s — they have a distinct form of the disease and are almost certain to develop its telltale brain plaques by age 65.

Recent research indicates that people with two copies of the APOE4 gene are almost certain to exhibit signs of the neurological disease by their mid-60s.