The researchers cautioned, though, that much more research remains to be done and that new treatments for many of these diseases are still years away.
The study is being published today in the British journal Nature. Its authors include Kenneth Fischbeck and Albert R. La Spada of Penn's medical school, along with researchers at the University of North Carolina and the National Hospital in London.
The study identified the genetic defect that causes Kennedy's disease, an uncommon illness that causes the spinal cord and brain stem to begin to deteriorate in men when they are about 30. Symptoms include the wasting away of muscles in the arms and legs, difficulty in speaking and swallowing and sometimes abnormal breast enlargement and reduced fertility.
Kennedy's disease is also known as spinal and bulbar muscular dystrophy. It affects fewer than 1,000 U.S. families, Stern said.
Fischbeck, an associate professor of neurology at Penn's medical school, said that the cause of the disease is a defect on the X chromosome that causes a segment of one of the genes to be about twice its normal size.
This defect causes the body to make proteins that are "toxic to the motor neurons, gradually killing them," he said.
Genes, the basic units of heredity, are found on 46 rodlike structures known as chromosomes that are found in the body's trillions of cells.
Fischbeck said that now that scientists know the cause of the disease there is a good chance that drugs can be found to combat it.
Robert Ross, vice president and executive director of the Muscular Dystrophy Association, said, "This finding means that diagnosis will be much more accurate, and it brings us much closer to finding effective treatments for the whole family of motor neuron diseases."
Donald Conwell, 65, a Willow Grove man who suffers from Kennedy's disease, said he did not believe that today's findings would have much impact on him.
"At my age I don't imagine that it will be much of a help," he said. ''But for the generations to come it will be tremendously helpful."
Conwell participated in the Penn study, which analyzed the genes in blood cells taken from 35 people who suffer from the disease. The scientists also examined 300 people who do not suffer from the disease.
The scientists found that the same genetic defect existed in all of the people who suffered from Kennedy's disease, while none of the people without the disease had the defect.
Conwell grew up in the Mount Airy section of Philadelphia and did not realize he had the disease until he was about 35 and he began to fall for no apparent reason.
"I'd be walking along the street and suddenly fall," he said. "At first I thought I had tripped over something. But then I realized that I didn't trip. I just fell."
His leg muscles gradually deteriorated, making it impossible for him to mow the lawn or do yardwork. He became disabled at the age of 54 and for the last 12 years has been confined to a wheelchair.