Genetic researchers have zeroed in on the location of a defective gene thought to be a contributing cause of a hereditary form of breast cancer, according to a new study published today.

Knowing where the gene is situated within the vast complement of human genes should make it possible soon to find the gene itself and develop a test that would let women know whether they have a predisposition to breast cancer. By itself, the defective gene does not cause cancer, but it does prime cells to turn cancerous when other mutations arise.

Scientists called the discovery a major step toward unraveling the genetic roots of breast cancer, especially for those families afflicted by the inherited form of the cancer, which in some families has stricken a mother, her daughters and her granddaughters.

James Watson, the Nobel prize winning biologist who heads Cold Spring Harbor Laboratory in New York, characterized the work by researchers at the University of California at Berkeley as "an extraordinary advance which should give great hope to women of this country."

While the gene the scientists seek has so far only been associated with an uncommon type of inherited breast cancer that strikes women early in life, researchers suspect the same gene may be involved in other forms of breast cancer. In such cases, the defect is not inherited but arises as a mutation in individuals.

In 1990, some 150,000 American women will be diagnosed with breast cancer and 44,000 will die. Though all forms of cancer are believed to result from a combination of several genetic mutations within a cell, relatively few of those mutant genes are passed from one generation to the next. About 10 percent of breast cancers may occur in women who inherit a mutant gene.

"The ultimate goal is to develop diagnostic techniques that will allow the detection of aberrant cells in the breast at the earliest stage," said Mary-Claire King, professor of epidemiology at Berkeley and the leader of the group that found the approximate location of the gene. King and her colleagues report their findings in today's issue of the journal Science.

Currently, women in families in which inherited early-onset breast cancer is common often elect to have their breasts removed before disease strikes, a radical treatment that in some cases may be unnecessary because a woman might not have inherited the gene.

"It happens every day," said David Page, a surgical pathologist at Vanderbilt University Medical School. "If we could tell who in these families {has} the changed gene, it would be terribly useful clinically."

"One of the most immediate needs is to identify those patients at risk," said Bert Vogelstein, professor of oncology at Johns Hopkins University School of Medicine. "This is a major breakthough in understanding this type of cancer."

The California researchers stress that the implicated gene has not yet been isolated. The scientific detectives succeeded only in finding where the defective gene lies, much like a private eye locating only the neighborhood where a villain resides, not his exact address. In this case, the mystery gene lies within a segment in the middle of the "long arm" of one member of the 17th pair of chromosomes. There are 23 pairs of chromosomes in a human's genetic package.

Now the race is on among a dozen groups in the United States, England, France and Iceland to find the gene.

In another study, also reported today in Science, researchers from the University of Utah Health Sciences Center found that a benign condition known as proliferative breast disease may predispose women to life-threatening breast cancer. In this condition, certain cells of the breast multiply at an abnormal rate without acquiring the ability to spread to other parts of the body and seed new tumors. Moreover, the Utah researchers, led by Mark Skolnick, found the tendency to develop proliferative breast disease appears to be inherited in some families.

Researchers believe that the progression of breast cancer may involve several genes acting in concert. For example, one gene may give rise to proliferative breast disease, while others lead the diseased tissue to tumor growth and spread.

Today's studies were both done using families with a high incidence of breast cancer. The gene's location was discovered by analyzing the DNA contained in white blood cells taken from 23 extended families, comprising more than 400 relatives, in which there were 146 cases of breast cancer.

"The toll on these families is horrific," King said. "These are women at enormously high risk."

King and her colleagues collected medical histories of the families and then used genetic probes to search through the DNA of each person for specific portions of genetic code, or markers, that showed up only among those who had breast cancer.

Now that they know where to look in detail, the researchers will attempt to find the gene itself and learn what special protein it produces and how the protein may start cells on their journey from normality to cancerous growth.