Scientists have succeeded in making early-stage sperm from human skin tissue, giving infertile men a shot at fathering children that are genetically their own.
The breakthrough could also lead to new contraceptives and a "miracle pill" to treat infertility.
However, the science is also fraught with moral and ethical concerns. Critics argue it's wrong to meddle with the building blocks of life and warn of a future in which babies are created through entirely artificial means.
Scientists have already succeeded in coaxing embryonic stem cells - master cells plucked from embryos in their first days of life - into turning into sperm. But using skin as a starting point would be more ethically acceptable. It would also mean the sperm would have the man's genes.
The American scientists used a cocktail of chemicals to wind back the "biological clock" in skin cells, turning them into cells with the chameleon-like qualities of embryonic stem cells.