In the Jewish community, the issue of genetic testing -- and the fear of being stigmatized -- has been a raw subject. Because Jewish women of European descent are so easy to screen for BRCA mutations, the gene is sometimes incorrectly referred to as the Jewish cancer gene.

This is the result of something called the founder effect rather than an increased susceptibility among Jewish women to cancer. Because Ashkenazi Jews have, until very recently, remained a cohesive, essentially closed group that by and large didn't marry outsiders, certain genetic markers can be traced back several thousand years to a handful of founder families. The same genetic phenomenon has been uncovered in Iceland, for precisely the same reasons; the integrity of the group has not been compromised by marriage to outsiders.

"The stigma issue in the Jewish community is totally misplaced," Offit says. "Not only does every group have its genetic baggage, but it turns out that in many cases what we thought was baggage is not. In the African-American community, for example, sickle-cell turns out to be the leftover result of the body protecting itself against malaria, which was widespread in certain parts of Africa before there was medication. So there may be a higher reason for BRCA as well that we just don't know about."

Once the external issues attached to genetic testing are stripped away, however, what remains is the extraordinary impact on the affected families and individuals. Mothers struggling with guilt over passing the genetic mutation to a child. The dynamic between two sisters when one tests positive and the other is negative. Families where certain relatives don't talk to one another and consequently can't communicate test results. Women who find the idea of having to tell anyone else in the family so painful they end up not even getting tested.

"I had an older woman with breast and ovarian cancer," says Myrna Ben-Yishay, a genetic counselor at Montefiore Medical Center/Albert Einstein College of Medicine, "who was simply unable to tell her two sons she was BRCA-positive. So she wrote them a letter that will be included as part of her will." (Men can carry and pass on the mutation. Though the impact of BRCA defects in men is uncertain, regular prostate and colon exams are urged.)

"I think that so far we have pretty much been dealing with a self-selecting group," says Ben-Yishay. "These women are unusual people. They are clearly women who believe that knowledge is power. We don't really know at this point how the average person will feel about all this because I think they're the people who haven't started to come in for genetic counseling yet."

Those people who should be tested, experts say, are women with multiple cases of breast or ovarian cancer in their families, particularly at an early age, generally defined as either premenopausal or before 50.

olly Ker's family, riddled with breast and ovarian cancer, certainly fits that description. Sitting in a Park Avenue South patisserie one morning recently, Ker, 27, pushes her fruit salad around in its bowl, sips her tea, and talks about having her ovaries removed.

Though she hasn't been tested yet, she has a plan. If she finds out she's positive later this year, when she figures she'll get tested, she intends to have an oophorectomy by the time she's 35. She worries about ovarian cancer because it's so insidious, but feels her regimen of frequent breast exams is sufficient to catch anything early enough to take care of it.

She wants to have kids, but she and her boyfriend aren't ready. "What worries me is I could be 32 and I won't feel any more ready and I've set this deadline. I've been raised in a generation of women who've been led to believe they can wait till they're 40 to have kids," says Ker.

A couple of weeks later, Ker, who works for an Internet company, sends me an e-mail saying her doctor found an abnormality in one of her breasts. He had her take an ultrasound. In the waiting room at the radiology lab, she thought for the first time about a mastectomy.