The secret of San Luis Valley: the discovery of a cancer gene among some Hispanic Catholics in southern Colorado supports the theory that they're descended from "secret Jews" who fled the Spanish inquisition.(San Luis Valley, New Mexico)

ONE SEPTEMBER DAY IN 2001, TERESA CASTELLANO, Lisa Mullineaux, Jeffrey Shaw and Lisen Axell were having lunch in Denver. Genetic counselors from nearby hospitals and specialists in inherited cancers, the four would get together periodically to talk shop. That day they surprised one another: they'd each documented a case or two of Hispanic women with aggressive breast cancer linked to a particular genetic mutation. The women had roots in southern Colorado, near the New Mexico border. "I said, 'I have a patient with the mutation, and she's only in her 40s,'" Castellano recalls. "Then Lisa said that she had seen a couple of cases like that. And Jeff and Lisen had one or two also. We realized that this could be something really interesting."

Curiously, the genetic mutation that caused the virulent breast cancer had previously been found primarily in Jewish people whose ancestral home was Central or Eastern Europe. Yet all of these new patients were Hispanic Catholics.

Mullineaux contacted Ruth Oratz, a New York City-based oncologist then working in Denver. "Those people are Jewish," Oratz told her. "I'm sure of it."

Pooling their information, the counselors published a report in a medical journal about finding the gene mutation in six "non-Jewish Americans of Spanish ancestry." The researchers were cautious about some of the implications because the breast cancer patients themselves, as the paper put it, "denied Jewish ancestry."

The finding raised some awkward questions. What did the presence of the genetic mutation say about the Catholics who carried it? How did they happen to inherit it? Would they have to rethink who they were--their very identity--because of a tiny change in the three billion "letters" of their DNA? More important, how would it affect their health, and their children's health, in the future?

Some people in the valley were reluctant to confront such questions, at least initially, and a handful even rejected the overtures of physicians, scientists and historians who were suddenly interested in their family histories. But rumors of secret Spanish Jewry had floated around northern New Mexico and the San Luis Valley for years, and now the cold hard facts of DNA appeared to support them. As a result, families in this remote high-desert community have had to come to grips with a kind of knowledge that more and more of us are likely to face. For the story of this wayward gene is the story of modern genetics, a science that increasingly has the power both to predict the future and to illuminate the past in unsettling ways.

Expanding the DNA analysis, Sharon Graw, a University of Denver geneticist, confirmed that the mutation in the Hispanic patients from San Luis Valley exactly matched one previously found in Ashkenazi Jews from Central and Eastern Europe. The mutation, 185delAG, is a variant of a gene called BRCA1. When normal and healthy, BRCA1 helps to protect breast and ovarian cells from cancer. An extremely …