Despite a new report warning that Ashkenazi Jews have a significantly higher genetic predisposition to breast cancer, researchers and activists say women should not panic.

"I don't think all women should run out and get tested," said Dr. Debu Tripathy, a medical oncologist in UCSF-Mount Zion Cancer Center's breast care division. "We don't quite know what to do with the information yet."

Women should temper their concern about the study's findings for several reasons:

*Non-hereditary factors lead to most breast cancers.

*Genetic testing is still limited to participants in clinical research.

*Women who test positive for the mutation cannot turn to any proven therapy to prevent the disease.

*Testing positive for the mutation doesn't actually predict the likelihood that a woman will develop breast cancer if she has no family history of it.

Despite these factors, researchers say the findings are important because they may lead to an understanding of what goes wrong inside a cell when it becomes cancerous.

"I think this is a very significant finding," Tripathy said.

The study's results, published in the October issue of the journal Nature Genetics, show that one in 100 Ashkenazi Jews in the United States has a particular genetic mutation that may predispose them to breast or ovarian cancer.

That mutation is at least three times — and possibly up to eight times — more prevalent than all known breast-cancer gene mutations combined within the general population.

Researchers suspect the mutation might account for up to 16 percent of breast cancer and 39 percent of ovarian cancer in Ashkenazi women under age 50.

The discovery was a joint effort by American and Israeli researchers, led by the National Institutes of Health, based in Maryland, and Hadassah Hospital, located in Jerusalem.

The findings are based on a study of blood samples from 858 unrelated American Jews whose ancestors were from Central or Eastern Europe. Eight of those Jews were found to have the same mutation on the so-called breast cancer gene.

None of the 815 non-Jews in a control group had the same defect.

According to the new study, a woman with the mutation and a family history of breast cancer has an 80 percent to 90 percent likelihood of getting breast cancer, and a 40 percent to 50 percent chance of getting ovarian cancer.

The mutation also may be linked to prostate cancer in men, and to colon cancer.

The new findings raise the question of whether all 6 million American Jews should be encouraged to test for the mutation. Between 80 percent and 90 percent of American Jews have Ashkenazi heritage.

The study's researchers do not support immediate widespread testing.

Neither does Dr. David R. Cox, a Stanford University professor of genetics and pediatrics who is part of a federal task force on the ethics of genetic testing.

If women test negative for the gene mutation, he said, they could have a false sense of security about their chances for breast cancer. At least 10 percent of the women who test negative may still develop breast cancer due to other genetic or environmental factors.

"For the vast majority of Jewish women who get breast cancer, it's not due to this particular change," Cox said.

If women test positive for the mutation, they still cannot prevent breast cancer — although they might become more vigilant with monthly self-exams and frequent mammograms. In addition, he said, the possibility exists that health insurers and employers could use a positive result to discriminate against women.

"I'm not an alarmist. But I do believe [testing] could screw up people's lives."

Jewish women who have had breast cancer appear to be split on the issue.

Patricia Barr, a National Breast Cancer Coalition board member who also sits on a federal task force on the ethics of genetic testing, doesn't believe she would have wanted to be tested for the mutation because she couldn't have done anything with the information.

"At this point, we don't know what a positive response really means," said Barr, a Vermont resident who was first diagnosed with the disease in 1987 and whose breast cancer is now in remission.

But Michelle Szychter, a Marin County nurse diagnosed with breast cancer last November, said she definitely would have wanted to be tested. Four women on her mother's side of the family had breast cancer, including her mother, aunt and two great-aunts.

Szychter, who is Ashkenazi, is almost convinced she has the mutation. With a positive test, she said, she might have had both breasts removed or started on an experimental anti-cancer drug.

But even Szychter, whose cancer is in remission, said that testing positive for the mutation has potential problems.

"I don't know if you'd want to tell your 20-year-old daughter to have a bilateral mastectomy," said Szychter, who lives in Greenbrae.

To determine whether widespread genetic testing should be the next step for Jews, two larger-scale studies are immediately getting under way.

The National Cancer Institute is conducting a study of 4,000 to 5,000 Jewish women and men in the Washington, D.C., and Baltimore areas. A similar one will be conducted in New York.

In recent years, scientists — and Jewish women — have noticed that breast cancer seems to be more prevalent among Jews than non-Jews. Some researchers have even included Jewish heritage as a risk factor for the disease.

However, some observers say that much of the breast cancer among Jews may not be due to ethnicity or religion but to the way Jewish women live.

Earlier studies found a 10 percent to 20 percent higher rate of breast cancer among Jewish women than non-Jewish women but suggested that the increase was based on diet and other non-hereditary factors.

Susan Weidman Schneider, editor-in-chief of the Jewish women's magazine Lilith, has run extensive articles on breast cancer twice in the past 18 months.

The current issue of Lilith includes three stories about Jewish rituals to help women deal with breast cancer.

"There's a real sense of doom, as if it's fated to happen," Schneider said. "I don't share that view."

Though the new study seems to have amplified Jewish women's anxieties about breast cancer, Schneider said the reaction among Lilith's staff to the possibility of testing for the mutation generally has been: "Who would want to know?…If nothing can be done, why know?