A Chinese scientist claims to have ushered in the age of genetic enhancement, announcing that he had tweaked the DNA of two human embryos, now twin girls, endowing at least one of them with resistance to HIV.

This kind of thing has always ended badly in movies, but like any advance in medical technology, it could help people and it could do harm.

If the claim is true, this is new territory on two fronts. It would be the first time scientists have changed the human germ line — the genes that could be passed down for centuries to come. And beyond that, it would be the first time that genetic engineering of any kind has been used for human enhancement rather than to correct a genetic disorder.

Though the gene editing technique the Chinese scientist used is relatively new — a technology known as Crispr — ethicists have been arguing over the prospect of genetically modified humans for decades. They’ve never come to much of a conclusion, except to advocate waiting until we know more about the safety.

Prominent researchers quoted by The Associated Press called the Chinese experiment “premature” and “unconscionable.” National Institutes of Health director Francis Collins issued a statement that germ line editing of embryos was “viewed almost universally as a line that should not be crossed.” But others saw potential benefits.

In late 2015, medical ethicist Art Caplan published an essay arguing that people should move past debating “whether” and move on to “how”: “We should be working to determine who decides when it is safe enough to be deployed, what counseling should be provided for parents considering its use, and how to broaden access for the poor.”

George Church, a geneticist at Harvard University and a co-inventor of Crispr, took a similar open-minded view of what the scientist claimed to have done in China. Yes, the aim was technically an enhancement, he said, but it was for a medical purpose, as opposed to some form of vanity. The end result, if it works, would be similar to getting a highly effective HIV vaccine with no need for booster shots.

He said he had corresponded directly with the team behind the scientist making the claim, who works for the University of Science and Technology in Shenzhen. He’s fairly convinced that the experiment was real, though the results have yet to be published in the open scientific literature. The mutation introduced — deleting a portion of a gene called CCR5 — does occur naturally and does make people resistant to HIV. One caveat, he said, is that the gene editing would need to affect both copies of the CCR5 genes to confer a benefit, and so far it looks like that happened in only one of the twins, while the other twin has just one copy of the gene alteration.

It makes sense that enhancement would be the goal of the first gene editing experiment. While there’s lots of research on Crispr gene editing to treat genetic diseases in people who were born with them, there’s little reason to use it to prevent inheritance of those diseases in the first place. For that, there are other, simpler methods, including making multiple embryos using in vitro fertilization, and testing them to make sure only ones without a disease mutation are implanted.

This new experiment could be seen as an extension of one conducted by nature. Scientists who trace the history of genes have discovered that within the last 3,000 years, a CCR5 mutation arose by chance somewhere in Europe. The mutation spread fast because people with two copies were protected from bubonic plague and smallpox. Scientists discovered this natural mutation in the late 20th century, when they realized those with two copies — about 10 percent of Europeans — were also protected from HIV.

But the new version of CCR5 didn’t spread to Africa or Asia. Now the Chinese have the chance to run their own experiment in HIV resistance.

But there is a downside, even if it worked perfectly. People with this mutation are more vulnerable to influenza and another class of diseases that includes West Nile virus.

Notions of perfecting the human race might crop up in Hollywood movies, but in real life it’s not on the horizon. Certainly at this moment in history, the technology is imperfect and trade-offs are inevitable.

Faye Flam is a Bloomberg Opinion columnist. She has written for The Economist, The New York Times, The Washington Post, Psychology Today, Science and other publications. She has a degree in geophysics from the California Institute of Technology.

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