Researchers studying large families in Utah say they have discovered the defective gene that causes a common and often fatal kidney disease, raising hopes for better diagnosis and perhaps even a cure, according to a report today.

The kidney disease known as Alport syndrome afflicts an estimated one in 5,000 U.S. residents, a rate similar to that of other inherited killers, such as muscular dystrophy and cystic fibrosis whose genes have also recently been isolated.

Most victims of Alport syndrome experience progressive kidney failure, often requiring a kidney transplant or prolonged kidney dialysis. The disease primarily attacks men, although women can carry the defective gene and pass it along to their children.

"It is very significant and a very important discovery," said Gladys Hirschman, director of the chronic renal disease program at the National Institute of Diabetes and Digestive and Kidney Diseases, the federal agency that funded much of the research.

The gene was discovered by researchers at the University of Utah and the University of Oulu in Finland. Their report appears in today's issue of the journal Science.

Normal genes are like instruction manuals telling the body how to develop and operate, but a mutated gene produces a garbled message. Alport syndrome is caused by mutations in a gene located on the X chromosome, the researchers found. Women have two copies of the X chromosome, while men have an X and a Y chromosome. This means that even if women inherit the defective gene on one X they would likely have a good copy of the gene on the other X chromosome.

In Alport syndrome, the defective gene fails to give the right instructions for the manufacture of a substance called collagen, which is used in the kidneys to make membranes important for filtering and cleaning blood. In Alport patients, the collagen membrane in the kidney deteriorates, frays and eventually scars, causing renal failure.

David Barker of the University of Utah, lead author of today's report, said new understanding of genetic defects may soon help physicians diagnose this disease with more precision. People at risk of carrying the defective gene could undergo screening to make informed decisions about whether to have children, he added.

Barker said further studies could also lead to new therapies for the kidney syndrome. No effective treatment is now available.