Many questions remain, of course. Most people do not have the protective gene mutation, but as common as Alzheimer’s is, most people do not get it. It is not clear why. And most who develop Alzheimer’s do not have one of the rare gene mutations that cause it. The reasons for their disease are unclear.

The discovery of the protective gene mutation, a product of the revolution that has taken place in genetics, arose when researchers scanned the entire DNA of 1,795 Icelanders.

About 1 in 100 had a mutation in the gene for a large protein that is sliced to form beta amyloid. Then the investigators studied people who had been given an Alzheimer’s diagnosis, and a group of people 85 and older. Those with the mutation appeared to be protected from Alzheimer’s disease.

The investigators, led by Dr. Kari Stefansson, chief executive at DeCode Genetics, an Icelandic company, looked at genomes of North Americans and found the gene mutation in only about 1 in 10,000 people. That indicates, Dr. Stefansson said, that the mutation arose relatively recently in Scandinavia.

The protective gene even appears to override a very strong risk factor for Alzheimer’s disease in old age — two copies of a gene known as ApoE4. Ninety percent of people with two ApoE4 genes get Alzheimer’s by age 80. But Dr. Stefansson says there are 25 people in his study with two copies of ApoE4. None have Alzheimer’s disease.

The research “is obviously right,” said John Hardy, an Alzheimer’s researcher at University College London and a discoverer of the first gene mutation found to cause the disease. “The statistics and the finding are pretty secure.”