Scientists in Oregon have created human embryos using the genes from three people.
Researchers at Oregon Health and Sciences University used a controversial technique that blended the genes of one man and two women.
The scientists stress they are not using the embryos to create children.
They believe the technique can lead to breakthroughs in replacing defective genes and preventing passing on of incurable diseases.
Critics warn it could lead to genetic manipulation and the creation of "designer babies." Others say problems might not surface for several generations.