LONDON, FEB. 15 -- British researchers said today they had pinpointed for the first time a possible genetic cause of Alzheimer's disease.

By studying two families prone to developing the degenerative brain disorder at an early age, the researchers said they were able to isolate a genetic mutation that alters a protein found in abnormally large quantities in the brains of people with the disease.

Alzheimer's disease is the fourth leading cause of death for adult Americans, and medical experts estimate that as many as one in 10 adults over the age of 65 suffer from it.

The British research team, from St. Mary's Hospital Medical School in London, said further investigation of other families would be needed to prove the gene was the cause. They cautioned that the mutation apparently causes only a small proportion of Alzheimer's disease cases.

However, they said it was the first cause of the disease to be identified.

The disease destroys brain cells, causing its victims to sink into progressive senility, leading to complete physical disability and death. There is no known cure and the disease cannot even be accurately diagnosed until death, when an autopsy of the brain reveals the damaged cells.

Between 10 and 30 percent of all victims suffer an inherited form of the disease called familial Alzheimer's disease, which can strike as early as the age of 30.