Scientists at the Oregon Health and Science University say they have successfully edited genes of human embryos in the first such attempt in the United States.
Previously, similar experiments have been reported only by scientists in China.
Engineering human genes in the embryo stage opens up the possibility of correcting their defective parts that cause inherited diseases. The new trait is passed on to subsequent generations.
But the practice is controversial, since many fear it could be used for unethical purposes such as creating "designer babies" with specific enhanced abilities or traits.
Oregon scientists led by Kazakhstan-born Shoukhrat Mitalipov successfully repeated the experiment on scores of embryos created with sperm donated for scientific purposes by men with inherited disease mutations.
The editing was done very close to the moment of fertilization of the egg in order to make sure the changes would be repeated in all subsequent cells of the embryo.
Scientists have been experimenting with gene editing for a long time, but the availability of the technique called CRISPR rapidly advanced the precision, flexibility and efficiency of cutting and replacing parts of the molecule chains that comprise genes.
Citing ethical concerns, the U.S. Congress made it illegal to turn genetically-edited embryos into babies. Many other countries do not have such regulations.