A Swedish medical researcher has taken another step toward eventually being able to engineer a custom-made human being. The experiment, first reported on NPR Thursday involves editing the genes of a developing human embryo.

The procedure by the biologist Fredrik Lanner at the Karolinska Institute in Stockholm is the first known attempt to modify such genes. It's been considered a taboo because of fears that it could step over ethical boundaries.

Many countries, such as the United States, allow gene-editing, but forbid implantation of such an embryo into the womb.

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Some U.S. states ban embryo research, but it would still be illegal anywhere in the U.S. to attempt to gestate a gene-edited embryo, according to Alta Charo, chair of a National Academy of Sciences panel developing guidelines on the ethics of human gene editing

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Gene-editing uses powerful new techniques such as CRISPR Cas-9 to snip out individual genes while the embryo is developing without affecting the rest of the genome. Parents who carry genes for an inherited disease such as Huntington's or cerebral palsy might want to have the ability to remove those genes so they could have healthy children, for example.

But some ethical experts have raised the scenario of so-called "designer babies" with editing to control genes that express traits of intelligence, height or certain facial features, for example.

"For me, the big ethical question doesn't arise until you try to make a baby out of it," said Henry T. Greely, director of the Center for Law and the Biosciences at Stanford University. "Trying to do research to understand embryonic development, there's good valid medical use for that kind of information."

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Still, Greely acknowledges that some scientists or the public might say that the Swedish experiment could be an ethical "slippery slope" toward a gene-edited human.

"Even if you don't intend to, it makes it easier for someone else to do it," Greely said.

Lanner is using embryos from an reproductive clinic near his lab in Stockholm. One expert wonders about the ethical controls over the work.

"What are the oversight and controls to prevent this technology from being misused and go to a stage that, for now, the scientific community has agreed is a no-go?" said Rosario Isasi, assistant professor at the University of Miami's department of human genetics.

In the United Kingdom, British scientists have gotten approval for similar gene-editing experiments with embryos in order to understand how they develop. A Chinese team attempted to do something similar a year ago on non-viable human embryos, but that experiment didn't work.

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Isasi noted that the Karolinska Institute was recently involved in a scandal involving a researcher who implanted artificial trachea into people seeded with their own stem cells. Two patients died and one was hospitalized, and an investigation revealed the doctor didn't follow ethical rules.

The researcher was fired and several faculty resigned from the institute, which is home to the Nobel prize committee.

"How did they supervise that research, which makes me wonder, what mechanisms were in place to oversee this (gene-editing) proposal," Isasi said.