Lately Vilain has been exploring molecular determinants of sex within the brain and whether they may be linked to gender identity. Despite classic dogma, he is certain that sex hormones do not drive neural development and behavioral differences on their own. SRY is expressed in the brain, he points out, suggesting that genes influence brain sexual differentiation directly. His lab has identified in mice 50 new gene candidates on multiple chromosomes for differential sex expression. Seven of them begin operating differently in the brain before gonads form. Vilain's group is testing these findings using mice and is collaborating with a clinic in Australia to study expression patterns of the sex-specific genes in transsexual people.

This work, like much of Vilain's efforts, treads on fairly touchy ground. He copes by sticking to his findings conservatively. "You also have to be aware of the social sensibilities," he explains. Accordingly, he has come to agree with some gender activists that it is time to revamp the vocabulary used to describe ambiguously sexed babies.

At the 2005 Intersex Consensus Meeting in Chicago, he stood before a group of 50 geneticists, surgeons, psychologists and other specialists and argued that terms such as "hermaphrodite," male or female "pseudohermaphrodite" and "intersex" were vague and hurtful. Instead of focusing on a newborn's confusing mix of genitals and gonads, he urged his colleagues to let the explosion of new genetic findings point toward a more scientific approach. Rather than using "hermaphrodite," for instance, he recommended referring to a "disorder of sexual development" (DSD) and applying the more precise term of "ovatesticular DSD."

Although the attendees eventually concurred, not everyone likes the new terminology. Some who prefer "intersex" feel that a "disorder" is demeaning. Milton Diamond, who studies sex identity at the University of Hawaii, complains that it stigmatizes people who have nothing wrong with their bodies.

But the decision to change nomenclature realizes a 15-year dream for Cheryl Chase, executive director of the Intersex Society of North America (ISNA). Chase has fought for years against secret, rushed surgeries intended to comfort parents and adjust anatomy to match an assigned social gender. Recalling how a doctor once called her "formerly intersex," she hopes physicians will begin to see mixed sex characteristics as a lifelong medical condition instead of a problem to be quickly fixed. "Now that we've accomplished the name change, culture can accomplish a little magic for us," she predicts.

For her, Vilain has been a valued ally in the process as a member of ISNA's medical advisory board. The job, he admits, forces him to listen to patients, a practice he considers unusual for the field. He expects the new, medicalized terminology for DSD to have what he describes wryly as "an interesting side effect," in which "medical science should apply" to clinical decisions about ambiguous sex.

Indeed, the new consensus statement on managing intersex disorders encourages physicians to see beyond a patient's sex organs, agrees conference co-organizer Peter A. Lee. The statement, released last fall, recommends speedy gender assignment but a more cautious approach to surgery. The family should participate in decision making, along with a multidisciplinary team of caregivers in specialties that include psychology and ethics. But Lee, a pediatric endocrinologist at the Penn State College of Medicine, cautions that much more work lies ahead to fill in data gaps. For instance, physicians have not measured how their choices affect patients over a lifetime.

On one Friday afternoon Vilain's white coat and stethoscope lay tossed amid the papers on his desk, a reminder that his discoveries have more than philosophical meaning. He sees six to eight patients in the U.C.L.A. intersex clinic every month, and in his on-call capacity, he receives two calls about babies in the hospital within the space of a couple of hours. Even while immersed in the workings of DNA transcription, Vilain stays grounded in what his findings mean for people's lives.

This article was originally published with the title Going beyond X and Y.

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