Scientists have, for the first time, successfully freed embryos of a piece of faulty DNA that causes deadly heart disease to run in families.
It potentially opens the door to preventing 10,000 disorders that are passed down the generations.
The US and South Korean team allowed the embryos to develop for five days before stopping the experiment.
The study hints at the future of medicine, but also provokes deep questions about what is morally right.
Science is going through a golden age in editing DNA thanks to a new technology called Crispr, named breakthrough of the year in just 2015.
Its applications in medicine are vast and include the idea of wiping out genetic faults that cause diseases from cystic fibrosis to breast cancer.
Darren Griffin, a professor of genetics at the University of Kent, said: "Perhaps the biggest question, and probably the one that will be debated the most, is whether we should be physically altering the genes of an IVF embryo at all.
"This is not a straightforward question... equally, the debate on how morally acceptable it is not to act when we have the technology to prevent these life-threatening diseases must also come into play."
The study has already been condemned by Dr David King, from the campaign group Human Genetics Alert, which described the research as "irresponsible" and a "race for first genetically modified baby".