Alzheimer's disease (AD) accounts for more than 50 percent of all cases of dementia. The most common form of Alzheimer's occurs in people older than sixty-five years, but there is also a form called early-onset Alzheimer's disease, or presenile dementia, which can begin as early as age forty.

Both forms of the disease have been linked to specific genetic abnormalities, at least in some people. Mutations in genes located on chromosomes 1, 14, and 21 have been identified in about 50 percent of patients with early-onset AD. In late-onset AD, scientists have discovered another possible cause, the apoE4 gene on chromosome 19. Studies have shown that people who inherit this defective gene from both parents are about eight times as likely to develop AD as people with two normal copies of the gene.

While the correlation between genetic defect and disease isn't perfect, genetic tests can tell whether a patient is at increased risk of developing the disease. However, with the availability of genetic tests for AD comes a huge dilemma for people concerned about getting the disease: Is it better to know one carries the defective gene, when all it means is that one is at increased risk of developing the disease and, more importantly, when the disease has no cure? Such is the case with Alzheimer's -- for now. Fortunately, a cure may be closer than once thought.

An innovative treatment first announced in 1999 has brought hope to countless AD sufferers and their families. For years, researchers had attempted to alter the course of the disease in a breed of mice genetically engineered to form amyloid plaques. These sticky protein fragments collect in the brains of human AD patients and may cause the brain damage associated with the disease.

The treatment that seems to have worked is a vaccinelike approach. Researchers injected the mice with a synthetic form of amyloid, called AN-1792, and produced an immune response similar to the response prompted by traditional vaccines. The treatment not only prevented amyloid plaques from forming in the brains of young mice, but also eliminated plaques already present in older mice.

Because scientists don't know for certain what role amyloid plays in AD, and because the mice do not model the disease completely, experts are cautious in their enthusiasm. Clinical trials in humans are now underway.