Alzheimer’s disease may be inherited more often than previously known, according to a new study that paints a clearer picture of a gene long known to be linked to the common form of dementia.

The authors of the study, published Monday in the journal Nature Medicine, say that this might even be considered a distinct, inherited form of the disease and that different approaches to testing and treatment may be needed.

Among people diagnosed with Alzheimer’s, researchers recognize familial forms of the disease and sporadic cases. Most cases are thought to be sporadic, which develop later in life. Familial forms, caused by mutations in any of three genes, tend to strike earlier and are known to be rare, accounting for about 2% of all Alzheimer’s diagnoses, or about 1 in 50 cases.

Under the new paradigm, 1 in 6 cases of Alzheimer’s would be considered to be inherited, or familial.

This shifting appreciation of inherited risk, researchers say, is due to a better understanding of the role of a fourth gene that carries the blueprints to make a lipid-carrying protein called apolipoprotein E, known as APOE. APOE ferries cholesterol throughout the body and brain and is thought to play a role in depositing or sweeping away sticky beta amyloid plaques, which are one hallmark of Alzheimer’s.

There are three types of the APOE gene a person can carry. One called APOE2 is thought to be protective against the development of Alzheimer’s disease. APOE3 is thought to confer a neutral risk of the disease.

APOE4, on the other hand, is bad news. It has long been recognized that people with at least one copy of the APOE4 gene have an elevated risk of developing Alzheimer’s disease, while people with two copies had a higher risk still.

Now, researchers say APOE4 shouldn’t just be recognized as a risk factor, it should be viewed as an inherited form of the disease, virtually assuring that a person who has two copies will get the biological changes associated with Alzheimer’s disease in their brains.