The creature genetic engineers fear most.

Whoever titled this week's conference on human germ-line modification "Babies by Design" was playing with fire. Maybe the event's host, the  Genetics and Public Policy Center, wanted some buzz or fun. It wouldn't be the first time genetic scientists have played this delicate game with the public: a bit of provocation, a bit of irony. They're flirting with us.

Like all flirts, the scientists fear that we'll take their winks and provocations the wrong way. They fear that we'll turn on them and ban their work, driven by misguided dread that they're creating some new subhuman or superhuman species. They don't fear such a species. They fear the species they already know: us.

Kathy Hudson, the center's founder, opens the conference in the amphitheater of a Washington, D.C., hotel. She warns the assembled biologists, lawyers, and ethicists that they won't call all the shots on genetics policy. The center is polling citizens, too, and it will incorporate their "values" in its recommendations. But as Hudson introduces the day's first presentation, it becomes clear that we non-scientists are poorly prepared to evaluate the new genetic technologies. They're too frighteningly counterintuitive.

The first presenter, Inder Verma of the Salk Institute, has brought a newspaper headline to poke fun at the media's ignorance. It suggests, erroneously, that the shrinking estimate of the number of human genes casts doubt on the importance of DNA. Around the room, scientists laugh. But I bet most people outside wouldn't get the joke, and they'd surely balk at Verma's proposal: to infect you with a virus that would rewrite your DNA. Many of us in the audience are nursing head colds, and this guy is telling us to turn over our genetic codes to a virus. On the projection screen, his prescription reads: "Tens of millions of recombinant viruses." Viruses aren't that bad, he explains. They don't want to kill your cells, since they need them to propagate. In fact, he and his colleagues have identified the perfect virus for the job. It's "a completely unexpected ally," he says. And then he tells us what it is: HIV.

See, here's where we run into a public opinion problem. Around most parts of the world, HIV has acquired—how shall we put it—a bad name. But as Verma explains, we've got it backward. HIV's amazing powers can be turned to good use. It can foist its genes on all kinds of cells. All we have to do is take out its bad genes and give it good ones to spread. And that's exactly what Verma and his colleagues have done: They've deleted 90 percent of the genome of HIV while preserving its ability to populate your cells with new genes. On the screen, Verma shows how an engineered virus gave mice a gene that stopped the production of plaques that cause Alzheimer's.

Curing Alzheimer's with HIV? Amazing. So what's holding us up? Well, there's a small problem. Viruses are sort of--you know--virulent. They want to infect you all over. Your doctor doesn't want that, because a genetic change that's helpful in one part of your body might be harmful in another. As the conference proceeds, we learn how to dress up this problem in the bloodless language of science. We hear about "insertional mutagenesis" and "aberrant activation of oncogenes." We hear of kids who got leukemia from gene therapy. Oops.

Not to worry, says the morning's second speaker. Leaning casually against the podium, George Daley of Harvard Medical School tells us about his two little boys. You could call them babies by design, he says, since they were tested by amniocentesis. They're three and six now. Daley tells us how he taught them to play chess and throw a baseball. See? Genetic screening isn't scary. Daley recently left the ivory tower to work at a hospital for children. He sees kids with horrible diseases. He sees adults with Cystic Fibrosis. The adults just want normal kids. Germ-line modification can help them. Is that so bad?

Daley proposes to fix genetic flaws in sperm and egg cells before they combine to produce children. The preliminary work has already been done in animals. Daley presses a button, and up comes a picture of a testicle in living tissue. I'm pretty sure it's a mouse testicle, though the magnification brings it a bit too close to home. And yes, that metallic line extending from the left of the screen into Downtown Testicle is—cross your legs, please--a needle. Hey, you have to inject those cells somehow. Suddenly the virus option doesn't look so bad.

The next idea on the table is cytoplasmic transfer. That's what fertility doctors do when they take the nucleus out of one woman's healthy egg and use the shell to host the nucleus from another woman's fragile egg. The new shell increases the odds of a successful pregnancy. But it isn't just a shell: It has everything but the nucleus, including cell components called mitochondria that contain DNA. Every couple that uses this fertility technique is putting foreign DNA into their kid. The amount is trivial, but the line has been crossed. Maybe we could do more with this technique, but the presenter worries that laypeople will freak out. They'll think it's messing around with embryos.

The conservatives in the audience seem to agree. They expect laypeople to take their side. Leon Kass, the chairman of the President's Council on Bioethics, stands up after the next presentation to ask council member Rebecca Dresser whether the public should be brought into the discussion before scientists adopt new genetic techniques. Dresser warns researchers that if they screw up any kids with genetic engineering, the ensuing P.R. disaster will lead to a shutdown of the field.