Kyle Orwig has been itching to do an experiment that would, in his words, “piss people off.” Orwig, a professor at the University of Pittsburgh, is an expert on the intricate biology of sperm cells—in particular, how specialized “stem” cells located in the male testes produce sperm. Every so often, however, a genetic flaw prevents these stem cells from completing this process, thus rendering the male infertile. The experiment Orwig has in mind is to use gene-editing technology to fix this flaw in the sperm-forming stem cells and then transplant them back into infertile mice, thereby demonstrating a potential treatment for male infertility.

It sounds simple enough, and, according to Orwig, it would be relatively straightforward to try—indeed, he has been transplanting sperm-forming stem cells into mice for 20 years. The consequences, however, could be momentous. The kind of experiment Orwig is contemplating would, if successful, push society right up to the brightest red line in contemporary biology: altering the genetic text of the human species in a way that is passed down to future generations.

If shown to be safe, effective and ethically acceptable, germ-line modification would confer unprecedented power on scientists—the power to edit the susceptibility to disease out of our species' DNA, for example, but also the power to manipulate human inheritance and “improve” the species, an aim that darkly harkens back to the eugenics movement of the early 20th century that reached its nadir in Nazi Germany.

Orwig, a broad-shouldered, burr-headed Oregonian who seems genial and determined, has no intention of crossing ethical lines. But he is something of a provocateur. By demonstrating that infertility in mice could be cured with a modest amount of genetic tinkering, he hopes to trigger a wider awareness that editing human genes is not an abstract, long-term technical challenge, as some have suggested, but a near-term possibility with practical medical consequences. Which is why Orwig recently told a colleague, “Let's just do this and piss some people off. Show them that it's possible, so nobody can say it's impossible. And get people talking about it.”

The issue of germ-line modification has assumed great urgency in the past two years because of a powerful gene-editing tool called CRISPR/Cas9, which allows scientists to alter the DNA of any organism—including, potentially, humans—with unprecedented precision and ease. In April 2015 Chinese researchers reported the first attempt to edit the genes of human embryos. The headlines—“Embryo Editing Sparks Epic Debate,” in Nature; “Eugenics Lurk in the Shadow of CRISPR,” in Science—signaled broad social unease. In the alarmist shorthand of media accounts, the possibility of gene editing triggered fears of “designer babies” and “genetic enhancement.”

The humble sperm cell, however, is a less controversial target. And whereas editing genes in an embryo remains a big challenge, many experts believe that approaching germ-line modification upstream of the embryo, in the sex cells that merge to form a zygote, is easier and arguably safer. Once you modify these cells, however, you are essentially modifying the human genome because the changes are permanently inscribed in the genetic text of the embryos they create. Orwig is among a handful of biologists who have experience genetically modifying and transplanting spermatogonial stem cells, the testicular cells that churn out sperm.

The field of reproductive medicine has a well-established track record for pushing headlong into the clinic with technological innovations. Infertility is also big business. If Orwig were to demonstrate in animals that a simple genetic fix is possible, it would be a tempting procedure for the tens of thousands of men who cannot make their own sperm, for whom options are currently limited, as well as for the in vitro fertilization (IVF) industry, which did an estimated $2 billion in business last year in the U.S. (and perhaps 10 times that amount worldwide).

Before administering any treatment, there must be proof that it works and causes no undue harm. Scientists would require such proof before even contemplating creating a human being with edited genes. Animal versions of those experiments are already under way, however, and the red line may be crossed soon. It could happen in China, where researchers have already taken the first tentative steps in editing human (albeit nonviable) embryos. It could happen in the U.K., where the government has legalized a form of germ-line modification known as mitochondrial replacement therapy and, this past February, approved gene-editing experiments on human embryos. And it could happen in an IVF clinic anywhere, building on recipes developed in laboratories such as Orwig's.

“This isn't theoretical,” Orwig says. “The mouse is here, and the human is not too far in the future. The pieces are already in place.”

Keep Calm and Carry On

The current debate over germ-line modification may feel familiar, but it treads on fundamentally new ground. Scientists began to acquire the Promethean ability to rewrite the language of heredity in the early 1970s, when biologists discovered that they could crudely cut and paste DNA with the use of enzymes harvested from bacteria, a technique called recombinant DNA. That advance caused unease about dangerous, genetically engineered microbes escaping the lab, prompting a voluntary—and unprecedented—moratorium on recombinant DNA research in 1974 and a historic meeting in 1975 of scientists at the Asilomar conference in California. Prominent molecular biologists such as David Baltimore, then at the Massachusetts Institute of Technology, debated the safety of the new technology, which led to federal guidelines governing the research. The Asilomar meeting was correctly viewed as a watershed cultural moment: Michael Rogers published a richly detailed account of “The Pandora's Box Congress” in Rolling Stone, and by the time the guidelines were in place, biotechnology emerged as one of the transformative industries of the 20th century.

Although society applauded the scientific community's decision in 1974 to hit the pause button on the headlong rush of research, many scientists felt it was an overreaction to hypothetical safety concerns. James D. Watson, co-discoverer of the DNA double-helix structure, called it “senseless hysteria.”

Since Asilomar, controversial biology has often landed in the public square with a bang, prompting big meetings with messy background noise. When the National Academy of Sciences debated recombinant DNA in 1977, protesters opposed to genetic engineering unfurled a banner quoting Adolf Hitler: “We Will Create the Perfect Race.” A meeting on human cloning in 2001 became a media circus. Maverick IVF doctors vowed to clone human babies. Television crews followed the would-be cloners everywhere (including to the bathroom). Wired magazine, in 2001, proclaimed on its cover: “Someone Will Clone a Human in the Next 12 Months.”

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Illustration by Jen Christiansen

This time around there is palpable unease among scientists, but they are also wary that another self-imposed moratorium might hold up progress. The result? Another meeting. Last December the National Academy of Sciences and the National Academy of Medicine co-hosted an international “summit” in Washington, D.C. (with the Royal Society and the Chinese Academy of Sciences). Baltimore acknowledged that altering human inheritance had remained “unthinkable” because of the cumbersome and inefficient first-generation tools of genetic engineering. “Over the years, however, the unthinkable has become conceivable,” he said, “and today we sense that we are close to being able to alter human heredity.” The overriding question, Baltimore continued, is, “How, if at all, do we as a society want to use this capability?”

The answer, to anyone who sat through the three-day meeting, as I did, seemed to be: we're not sure, but there's plenty of time to think it through. Numerous talks, including a plenary address by genome scientist Eric Lander of the Broad Institute of M.I.T. and Harvard University, stressed the technical hurdles and the lack of compelling medical needs for human germ-line modification in the near future. “It might, might, might be a good idea that before we make permanent changes to the human gene pool,” Lander warned, “we should exercise considerable caution.”

The organizers deftly steered clear of an Asilomar-like moratorium. Baltimore read a carefully worded statement from the meeting organizers acknowledging that it would be “irresponsible” to pursue human germ-line editing in the clinic at this time. At the close of the summit, he went on to explain that the organizers had deliberately avoided calling for a ban or moratorium. “Neither word did we want to use,” he said. “Neither word have we used.” Basic research could, and should, proceed unimpeded, but the public need not be concerned about imminent developments: human applications of germ-line editing were implausible, unnecessary, unwise and certainly not right around the corner.

That is not the way everyone in the scientific community sees it. The organizers of the Washington meeting framed the issue as “when, if at all.” But a different word often crops up in private conversations with biologists when you ask about the prospects for germ-line editing. That word is “inevitable.”

The Timeline

Some scientists viewed the National Academies meeting as an effort to “reinforce the status quo,” according to biologist George Church of Harvard Medical School. “They basically want the public to calm down,” he says. “That was their goal. And no matter what we said, that was going to be the goal. I don't want to stir [the public] up, and I don't want to calm them down, either. I want them to have an accurate view of where things are going.” And the public needs to start thinking about gene-editing the human genome now, Church says, because science is already bumping up against the red line.

Despite a thicket of international provisions regulating human embryo research, Church and others believe that the creation of gene-edited sex cells in the test tube (the technical term is “in vitro gametogenesis,” or IVG) has made great strides in recent years without attracting the same public scrutiny, or provoking the same ethical discomfort, as gene editing in embryos.

“In terms of the technology, that's ready to go now,” says I. Glenn Cohen, a bioethicist at Harvard Law School. “In vitro gametogenesis is much closer to the mark than any other way to do it.” And Ina Dobrinski, an expert in reproductive biology at the University of Calgary who works on gene editing of large animals such as pigs, adds, “Theoretically, we can do it. Practically [speaking], nobody is even touching it because of the ethical issues.”

If human germ-line editing is inevitable, despite the ethical concerns (and indeed legal prohibitions in many countries), how might it happen? Speculation among biologists has become something of a parlor game, but I turned to Church, a card-carrying futurist, to lay out a plausible scenario. He was happy to oblige.

Church sees the germ-line Rubicon being crossed because sperm do not seem to arouse the same ethical passions as embryos or even egg cells. (Bioethicist Cohen agrees: “People do not believe that masturbation is genocide.”) He also thinks gene therapy, not CRISPR per se, will set the table for this momentous change because it is already accepted: the U.S. Food and Drug Administration has allowed many trials of gene therapy in somatic (nongerm-line) cells. “Gene therapy is happening in young children already and will happen in younger and younger children,” Church says.

In a highly publicized case last fall, for example, British researchers used gene-editing techniques to alter the immune cells of an infant battling leukemia. And the leap to germ-line gene therapy, according to Church, will occur not in human embryos but in the most proletarian, plentiful and expendable cell in the human body: sperm. Gene-editing sperm will spare couples the agony of destroying IVF embryos that, on preimplantation screening, are shown to possess variants that forecast some devastating single-gene disorders, he believes. “Maybe half the people in the U.S. already feel that they're not comfortable killing embryos, but I think people would be comfortable genetically altering sperm,” he observes. “No embryos die.”

Two obvious targets, Church adds, would be single-gene disorders (such as Tay-Sachs disease) and infertility. “You could also do it in human spermatogonial stem cells,” he says, referring to the specialized adult stem cells in male testes that generate sperm cells—millions on millions of mindless, headlong swimmers—every day. “People don't really care about spermatogonia. Most people don't even know how to pronounce it. So they're going to let you mess around with them, right?” Church continues. “You will be able to do all kinds of things to show that they are functionally normal—that you've taken the sperm that can't swim, and that you've now taken their stem cells and made sperm that can swim. You can test that in a lab without any eggs being involved. And then in the fertility clinic, the dad will say, ‘Hey, those are pretty awesome sperm. Let's try them out and see what they can really do.’ And I don't know who's going to stop them from doing that.”

As for the timeline, Church says, “I think there will be multiple clinical infertility solutions involving gene therapy soon.”

How soon?

“The next couple years,” he says. “It would be very hard to resist.”

In his talk at the National Academies meeting, Orwig flashed a slide that said: “Germ-Line Gene Therapy Is Technically Feasible Today.” Afterward, according to Orwig, a member of the planning committee sidled up to him backstage and said, “Germ-line gene therapy is going to happen, I guarantee it.” That sentiment never made it into the final meeting communiqué. But it galvanized Orwig.

“Whereas it might have been something I wanted to do hiding under a rock, it's now like, ‘You go, man!’ Let me get to work and prove to you that I can do it.”

In animals, of course.

A Gentle Nudge Down the Slope

A few steps down the hallway from Orwig's office is a complex of rooms that houses hundreds of mice. You have to don a gown and booties and a mask to enter—not because you might catch something from the mice but because they might catch something from you. Many of the cages contain what are called nude mice—pink, wrinkled little rodents that resemble scrotums with eyes and feet. They are nude in the sense that they were bred to have compromised immune systems that accommodate cells transplanted from other species—for instance, human spermatogonial stem cells with mutations—to allow researchers to better understand the biology of male infertility.

If, as Church says, “everything is going to be done in animals first,” the road to human germ-line modification runs through rooms like these. CRISPR makes the task more efficient (“It is so freaking easy!” Orwig says), but scientists have been able to alter the genes of sperm-making cells for more than two decades, beginning in 1994, when University of Pennsylvania biologist Ralph Brinster (Orwig's mentor) did the pioneering experiments in mice.

Male infertility has many causes, including obstructive “plumbing” issues, glitches in the incredibly complex process of sperm creation, and underachieving sperm. But in many cases, males simply can't make sperm at all; the condition, known as nonobstructive azoospermia, affects roughly 350,000 men in the U.S., according to Orwig. Several genes have been associated with the failure to produce sperm, including tex11 and sohlh1, and those cases form the backdrop of the experiment Orwig is eager to do.

What Orwig wants to do is take infertile mice, which have a dysfunctional version of one of these genes, remove the sperm-forming stem cells from their testes and correct the defect in those cells by using the new gene-editing techniques. Once the altered stem cells are grown to sufficient numbers in the test tube and screened for precisely the correct alteration, they can be transplanted back into the testes of the animals. And at least in animal experiments of this kind, there is no need for any fancy molecular tests—if the gene editing is successful, Orwig will know within a couple of months because the infertile males will unambiguously demonstrate their ability to become fathers.

“We've been transplanting stem cells for 25 years in almost every species—mice, rats, hamsters, sheep, goats, pigs, dogs and monkeys,” Orwig says. “That's a pretty broad swath of evolution, and in all this time, in all these animals, as far as we know, nothing bad has happened.” That is why Orwig is optimistic he can demonstrate that editing the genes of stem cells in mice can reverse infertility.

This may seem like an innocuous animal experiment, but to edit a sperm-forming stem cell is to permanently modify the germ line because the resulting sperm cells pass the correction along to the next generation. A potential treatment for male infertility would cross the red line. And although Orwig has no plans to do the obvious human follow-up in his Pittsburgh lab, a successful preclinical demonstration in mice and primates would provide the impetus for an attempt in the private sector—which is where Church believes the final steps will unfold. “Sperm-editing efforts will be privately funded,” he says, “just like other therapies.”

Developing such a clinical treatment would face technical hurdles, of course. For one thing, scientists would have to find a way to maintain human spermatogonial stem cells long enough to select the right ones for transplantation—still not a trivial task. But these male stem cells offer much less of a moving target than embryos, which are dynamic and change rapidly. The Chinese researchers who have attempted gene editing in embryos with CRISPR, for example, have reported both “untoward mutations” and “mosaicism,” meaning that some cells in the embryos show successful editing, whereas others do not. Moreover, the DNA of gene-edited stem cells can be screened before an embryo is produced.

That is what makes Orwig's potential mouse experiment so politically inconvenient. Because of prohibitions enacted by Congress in the 1990s, the National Institutes of Health cannot fund any research that involves the destruction of human embryos. A human version of Orwig's proposed mouse experiment might sidestep that prohibition, but it would probably fall under a new obstacle that the House of Representatives introduced two weeks after the December summit on gene editing. In a two-sentence passage buried in the 2,009-page omnibus spending bill of 2015, Congress inserted language forbidding the FDA to consider any medical intervention relying on the use of gene-edited embryos; the wording does not explicitly prohibit editing germ cells, but Stanford University law professor Henry Greely believes “the FDA would take the position that those sperm were more than minimally manipulated human cells that would require FDA approval as a drug or biological product.” The regulatory piece, he thinks, could add a decade or two to Church's timeline.

That does not mean Orwig's mouse experiment would be against the law—just a gentle nudge down the slippery slope toward germ-line modification. The step across the red line could happen in private IVF clinics, which have a long (and blemished) history of pushing the envelope on new reproductive technologies. “It's such an easy technology to apply that it would only take somebody with a little chutzpah to get together with someone in an IVF clinic and, you know, take a shot at it,” says George Daley, a stem cell biologist at Boston Children's Hospital. “This is coming down the pike, and people need to start thinking about it,” he notes. “This is a potentially disruptive reproductive technology.”

It probably will not happen in the U.S. unless the public—and political—perception of germ-line modification becomes more accommodating, but Orwig is quietly preparing for that day. “We're going to work real hard behind the scenes,” he says, “until the worldview changes.”

Crossing Borders

The “worldview” on germ-line editing is complicated and contradictory. A majority of Americans do not like the idea of editing genes in either embryos or germ cells, according to a recent analysis of 17 public opinion polls published in the New England Journal of Medicine. Yet paradoxically, most people support gene editing in adults “aimed at preventing one's children from inheriting certain diseases.” (Robert J. Blendon, lead author of the study, says any intervention on the adult side that is positive for the next generation, including germ cells, would have “considerable public support.”) Moreover, the NEJM study also pointed out that many of these public opinion polls pose their questions using language that “might not be scientifically precise.” In other words, although the National Academies meeting adjourned last December with a pledge to continue the public conversation about germ-line editing, it is not clear that the public even understands the terms of that conversation. And while public forums struggle to find an effective vocabulary, the science races ahead.

As we spoke in his office last spring, Orwig nodded at a scientific reprint sitting on his desk. “I really, really love this paper,” he said. He was referring to research published this past February in the journal Cell Stem Cell by a group headed by Qi Zhou of the Chinese Academy of Sciences. The experiment basically provided a recipe for the in vitro creation of germ cells.

The researchers showed that they could create sperm-forming stem cells in a dish; with a technique currently used in IVF clinics, these cells could be injected into egg cells to create fertile male mice. Harvard's Daley says of this advance: “With the addition of CRISPR, you've got the brave new world.”

When Aldous Huxley imagined his brave new world in 1932, the story unfolded under one totalitarian regime, with neither national boundaries nor local regulations. In today's world, germ-line editing in any one place means the germ line is edited everywhere. “Regulation is country-specific, but science crosses borders,” Harvard Law's Cohen says. Even if there were laws against germ-line modification in the U.S., you would have to build a wall much higher than the one proposed by Donald Trump to keep American germ lines insulated from an eventual influx of modified DNA.

“If you play out the world to 100 years from now, if anybody does this anywhere, that's the end game,” Cohen says. “Over time those people will mate and create offspring and will cross borders and enter our shores. And if the safety and efficacy are worked out, it's inevitable that you're going to have people walking around in the world, and they're going to be reproducing, and they will end up in this country, and those changes will enter the U.S. gene pool.”

As I am concluding my visit with Orwig, he glances at the computer on his desk. A reporter has sent an e-mail seeking comment on yet another experiment cozying up to the red line: a group in China has just reported its attempt to edit human embryos (nonviable) to be resistant to HIV infection. “Eventually we'll learn a vocabulary that acknowledges that we're there,” Orwig says. “But I feel we're already there.