Cystic fibrosis has become one of the latest in a growing list of diseases for which a "genetic marker" has been found, moving researchers a step closer to finding the cause of the most common fatal hereditary disease.

The marker, which signals the presence of a disease-causing gene, is a very crude one and cannot be used to make an early diagnosis of cystic fibrosis. Instead, it will make the next steps easier: locating the chromosome on which the disease-causing gene may be found, locating the disease-causing gene itself, and eventually making an accurate diagnostic test. New treatments also may develop out of that chain of work.

The genetic "marker" has been found by teams led Dr. Helen Donis-Keller at Colloborative Research Inc. of Boston and Drs. Manuel Buchwald and Lap-Chui Tsee at the Hospital for Sick Children in Toronto.

Cystic fibrosis strikes about one in 2,000 infants, and has about 30,000 victims in the nation. It is a degenerative pulmonary disease whose victims have a life expectancy of 21 years. It is believed to be caused by an abnormality of single gene among the more than 100,000 humans inherit.

A marker is a random piece of the long strands of DNA that make up our total inheritance. It is not a cause of disease. But it can be spotted consistently using tests called "DNA probes." Working backward, researchers can use such identifiable bits of DNA to see if any are consistently inherited together with a disease.

If the marker and disease are inherited together with a high frequency, it suggests they are close together on the individual's DNA . If they are close enough together, the marker can signal the presence of the nearby disease-causing gene. If it is less often linked, it can still suggest a region in which the disease-causing gene may be found.

The step taken by researchers has now cut down the territory in which scientists have to search to find the CF-causing gene. While not close enough to call it diagnostic, the marker is close enough to the disease gene to eliminate about 99 percent of the DNA as possible sites for the CF-causing gene.

"Looking for the gene is like looking for a house in a city without knowing the street address," said Buchwald. "Now our research results can point us in the right direction."

Officials of the Cystic Fibrosis Foundation said the work was a major advance in discovering the CF gene, but also cautioned that the gene itself has not yet been found and no diagnostic test is likely to come out of the work.

Cystic fibrosis afflicts mainly whites and causes the body to make abnormally large amounts of mucus that clogs the lungs and fouls the pancreas. The disease becomes progressively worse, and although most victims in the past have died early in childhood, current treatments to clear the lungs often allow victims to live into their teens and twenties.