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 genesis—one 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, she’d been a key player in developing a powerful new technology for editing DNA, called CRISPR-Cas9. With Church, Yang had founded a small biotechnology company 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 didn’t hesitate. Yes, of course, she said. In fact, the Harvard laboratory had a project under way 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 correct disease genes and 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, Alzheimer’s, and, Yang told me, maybe the effects of aging. Such history-making medical advances could be as important to this century as vaccines were to the last.

That’s the promise. The fear is that germ-line engineering is a path toward a dystopia of superpeople 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 tomorrow’s leaders and scientists?

Just three years after its initial development, CRISPR technology 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. It’s so precise that it’s expected to turn into a promising new approach for gene therapy 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 wouldn’t affect germ cells, and the changes in the DNA wouldn’t get passed to future generations.

In contrast, the genetic changes created by germ-line engineering would be passed on, and that’s what has 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 Union’s convention on human rights and biomedicine says tampering with the gene pool would be a crime against “human dignity” and human rights.

But all these declarations were made before it was actually feasible to precisely engineer the germ line. Now, with CRISPR, it is possible.

CRISPR – Part 1- GENETIC MANIPULATION –The Springboard CRISPR – Part 2- The DESTROYER CRISPR – Part 3- THE ULTIMATE END CRISPR – Part 4 – UNLEASHED

Yang would later tell me that she dropped out of the project not long after we spoke. Yet it remained difficult to know if the experiment she described was occurring, canceled, or awaiting publication. Sinclair said that a collaboration between the two labs was ongoing, but then, like several other scientists whom I’d asked about germ-line engineering, he stopped replying to my e-mails.

Regardless of the fate of that particular experiment, human germ-line engineering has become a burgeoning research concept. At least three other centers in the United States are working on it, as are scientists in China, in the U.K., and at a biotechnology company called OvaScience, based in Cambridge, Massachusetts, that boasts some of the world’s leading fertility doctors on its advisory board.

The objective of these groups is to demonstrate that it’s possible to produce children free of specific genes involved in inherited disease. If it’s possible to correct the DNA in a woman’s egg, or a man’s sperm, those cells could be used in an in vitro fertilization (IVF) clinic to produce an embryo and then a child. It might also be possible to directly edit the DNA of an early-stage IVF embryo using CRISPR. Several people interviewed by MIT Technology Review said that such experiments had already been carried outin China and that results describing edited embryos were pending publication. These people, including two high-ranking specialists, didn’t wish to comment publicly because the papers are under review.

All this means that germ-line engineering is much further along than anyone imagined. “What you are talking about is a major issue for all humanity,” says Merle Berger, one of the founders of Boston IVF, a network of fertility clinics that is among the largest in the world and helps more than a thousand women get pregnant each year. “It would be the biggest thing that ever happened in our field.” Berger predicts that repairing genes involved in serious inherited diseases will win wide public acceptance but says the idea of using the technology beyond that would cause a public uproar because “everyone would want the perfect child”: people might pick and choose eye color and eventually intelligence. “These are things we talk about all the time,” he says. “But we have never had the opportunity to do it.”

Editing embryos

How easy would it be to edit a human embryo using CRISPR? Very easy, experts say. “Any scientist with molecular biology skills and knowledge of how to work with [embryos] is going to be able to do this,” says Jennifer Doudna, a biologist at the University of California, Berkeley, who in 2012 co-discovered how to use CRISPR to edit genes.

To find out how it could be done, I visited the lab of Guoping Feng, a biologist at MIT’s McGovern Institute for Brain Research, where a colony of marmoset monkeys is being established with the aim of using CRISPR to create accurate models of human brain diseases. To create the models, Feng will edit the DNA of embryos and then transfer them into female marmosets to produce live monkeys. One gene Feng hopes to alter in the animals is SHANK3. The gene is involved in how neurons communicate; when it’s damaged in children, it is known to cause autism.

Feng said that before CRISPR, it was not possible to introduce precise changes into a primate’s DNA. With CRISPR, the technique should be relatively straightforward. The CRISPR system includes a gene-snipping enzyme and a guide molecule that can be programmed to target unique combinations of the DNA letters, A, G, C, and T; get these ingredients into a cell and they will cut and modify the genome at the targeted sites.

But CRISPR is not perfect—and it would be a very haphazard way to edit human embryos, as Feng’s efforts to create gene-edited marmosets show. To employ the CRISPR system in the monkeys, his students simply inject the chemicals into a fertilized egg, which is known as a zygote—the stage just before it starts dividing.

Feng said the efficiency with which CRISPR can delete or disable a gene in a zygote is about 40 percent, whereas making specific edits, or swapping DNA letters, works less frequently—more like 20 percent of the time. Like a person, a monkey has two copies of most genes, one from each parent. Sometimes both copies get edited, but sometimes just one does, or neither. Only about half the embryos will lead to live births, and of those that do, many could contain a mixture of cells with edited DNA and without. If you add up the odds, you find you’d need to edit 20 embryos to get a live monkey with the version you want.

That’s not an insurmountable problem for Feng, since the MIT breeding colony will give him access to many monkey eggs and he’ll be able to generate many embryos. However, it would present obvious problems in humans. Putting the ingredients of CRISPR into a human embryo would be scientifically trivial. But it wouldn’t be practical for much just yet. This is one reason that many scientists view such an experiment (whether or not it has really occurred in China) with scorn, seeing it more as a provocative bid to grab attention than as real science. Rudolf Jaenisch, an MIT biologist who works across the street from Feng and who in the 1970s created the first gene-modified mice, calls attempts to edit human embryos “totally premature.” He says he hopes these papers will be rejected and not published. “It’s just a sensational thing that will stir things up,” says Jaenisch. “We know it’s possible, but is it of practical use? I kind of doubt it.”

For his part, Feng told me he approves of the idea of germ-line engineering. Isn’t the goal of medicine to reduce suffering? Considering the state of the technology, however, he thinks actual gene-edited humans are “10 to 20 years away.” Among other problems, CRISPR can introduce off-target effects or change bits of the genome far from where scientists had intended. Any human embryo altered with CRISPR today would carry the risk that its genome had been changed in unexpected ways. But, Feng said, such problems may eventually be ironed out, and edited people will be born. “To me, it’s possible in the long run to dramatically improve health, lower costs. It’s a kind of prevention,” he said. “It’s hard to predict the future, but correcting disease risks is definitely a possibility and should be supported. I think it will be a reality.”

Editing eggs

Elsewhere in the Boston area, scientists are exploring a different approach to engineering the germ line, one that is technically more demanding but probably more powerful. This strategy combines CRISPR with unfolding discoveries related to stem cells. Scientists at several centers, including Church’s, think they will soon be able to use stem cells to produce eggs and sperm in the laboratory. Unlike embryos, stem cells can be grown and multiplied. Thus they could offer a vastly improved way to create edited offspring with CRISPR. The recipe goes like this: First, edit the genes of the stem cells. Second, turn them into an egg or sperm. Third, produce an offspring.

Some investors got an early view of the technique on December 17, at the Benjamin Hotel in Manhattan, during commercial presentations by OvaScience. The company, which was founded four years ago, aims to commercialize the scientific work of David Sinclair, who is based at Harvard, and Jonathan Tilly, an expert on egg stem cells and the chairman of the biology department at Northeastern University (see “10 Emerging Technologies: Egg Stem Cells,” May/June 2012). It made the presentations as part of a successful effort to raise $132 million in new capital during January.

During the meeting, Sinclair, a velvet-voiced Australian whom Time last year named one of the “100 Most Influential People in the World,” took the podium and provided Wall Street with a peek at what he called “truly world-changing” developments. People would look back at this moment in time and recognize it as a new chapter in “how humans control their bodies,” he said, because it would let parents determine “when and how they have children and how healthy those children are actually going to be.”

The company has not perfected its stem-cell technology—it has not reported that the eggs it grows in the lab are viable—but Sinclair predicted that functional eggs were “a when, and not an if.” Once the technology works, he said, infertile women will be able to produce hundreds of eggs, and maybe hundreds of embryos. Using DNA sequencing to analyze their genes, they could pick among them for the healthiest ones.

Genetically improved children may also be possible. Sinclair told the investors that he was trying to alter the DNA of these egg stem cells using gene editing, work he later told me he was doing with Church’s lab. “We think the new technologies with genome editing will allow it to be used on individuals who aren’t just interested in using IVF to have children but have healthier children as well, if there is a genetic disease in their family,” Sinclair told the investors. He gave the example of Huntington’s disease, caused by a gene that will trigger a fatal brain condition even in someone who inherits only one copy. Sinclair said gene editing could be used to remove the lethal gene defect from an egg cell. His goal, and that of OvaScience, is to “correct those mutations before we generate your child,” he said. “It’s still experimental, but there is no reason to expect it won’t be possible in coming years.”

…Bermingham told me he never imagined he’d have to be taking a position on genetically modified babies so soon. Modifying human heredity has always been a theoretical possibility. Suddenly it’s a real one. But wasn’t the point always to understand and control our own biology—to become masters over the processes that created us?

Doudna says she is also thinking about these issues. “It cuts to the core of who we are as people, and it makes you ask if humans should be exercising that kind of power,” she told me. “There are moral and ethical issues, but one of the profound questions is just the appreciation that if germ-line editing is conducted in humans, that is changing human evolution.” One reason she feels the research should slow down is to give scientists a chance to spend more time explaining what their next steps could be.

“Most of the public,” she says, “does not appreciate what is coming.”

This story was updated on April 23, 2015