If anyone had devised a way to create a genetically engineered baby, I figured George Church would know about it.

At his labyrinthine laboratory on the Harvard Medical School campus, you can find researchers giving E. Coli a novel genetic code never seen in nature. Around another bend, others are carrying out a plan to use DNA engineering to resurrect the woolly mammoth. His lab, Church likes to say, is the center of a new technological genesisone in which man rebuilds creation to suit himself.

When I visited the lab last June, Church proposed that I speak to a young postdoctoral scientist named Luhan Yang, a Harvard recruit from Beijing whod been a key player in developing a new, powerful technology for editing DNA called CRISPR-Cas9. With Church, Yang had founded a company called eGenesis to engineer the genomes of pigs and cattle, sliding in beneficial genes and editing away bad ones.

As I listened to Yang, I waited for a chance to ask my real questions: Can any of this be done to human beings? Can we improve the human gene pool? The position of much of mainstream science has been that such meddling would be unsafe, irresponsible, and even impossible. But Yang didnt hesitate. Yes, of course, she said. In fact, the laboratory had a project to determine how it could be achieved. She flipped open her laptop to a PowerPoint slide titled Germline Editing Meeting.

Here it was: a technical proposal to alter human heredity.

Germ line is biologists jargon for the egg and sperm, which combine to form an embryo. By editing the DNA of these cells or the embryo itself, it could be possible to eliminate disease genes and to pass those genetic fixes on to future generations. Such a technology could be used to rid families of scourges like cystic fibrosis. It might also be possible to install genes that offer lifelong protection against infection, Alzheimers, and, Yang told me, maybe the effects of aging. These would be history-making medical advances that could be as important to this century as vaccines were to the last.

The fear is that germ line engineering is a path toward a dystopia of super people and designer babies for those who can afford it.

Thats the promise. The fear is that germ line engineering is a path toward a dystopia of super people and designer babies for those who can afford it. Want a child with blue eyes and blond hair? Why not design a highly intelligent group of people who could be tomorrows leaders and scientists?

CRISPR was discovered only three years ago but is already widely used by biologists as a kind of search-and-replace tool to alter DNA, even down to the level of a single letter. Its so precise that its widely expected to turn into a promising new approach for gene therapy treatment in people with devastating illnesses. The idea is that physicians could directly correct a faulty gene, say, in the blood cells of a patient with sickle-cell anemia (see Genome Surgery). But that kind of gene therapy wouldnt affect germ cells, and the changes in the DNA wouldnt get passed to future generations.

In contrast, the genetic changes created by germ line engineering would be passed on, and thats what has always made the idea seem so objectionable. So far, caution and ethical concerns have had the upper hand. A dozen countries, not including the United States, have banned germ line engineering, and scientific societies have unanimously concluded that it would be too risky to do. The European Unions convention on human rights and biomedicine says tampering with the gene pool would be a crime against human dignity and human rights.

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Engineering the Perfect Baby