Laboratory experiments suggest it may be possible one day to genetically alter human sperm cells to permanently eradicate genetic diseases.
The sperm were genetically modified
Japanese and US researchers managed to insert foreign DNA into zebrafish sperm cells - then successfully mature them into working sperm.
Previous attempts have led to offspring with a mixed genetic identity.
However, attempts to tinker with the human "germline" are considered fraught with danger by most scientists.
This is because of the danger of inadvertantly introducing genetic problems which then persist from generation to generation.
The UK's fertility watchdog says no-one has tried to do it here - there is likely to be strong opposition to any attempt.
So far, gene therapy is restricted to treatments which try to insert new genes into adult human cells, rather than ones which attempt to correct the problem permanently by inserting them into germ cells.
The research team, from Fukui Prefectural University and the National Human Genome Institute in Bethesda, US, wrote up their experiment in the journal Proceedings of the National Academy of Sciences.
They infected immature zebrafish germ cells with viruses designed to insert a particular gene.
Afterwards, using combinations of laboratory solutions, they managed to induce these germ cells to mature into sperm themselves.
More than 1,000 zebrafish eggs were then exposed to these modified sperm and 104 eggs were fertilised.
Of these, only five of the 89 offspring carried the gene, but experts still believe that this is a significant advance.
Dr Shawn Burgess, from the National Human Genome Institute, said: "To our knowledge this is the first time that sperm cells have been cultured entirely in vitro and used to produce a transgenic animal."
No go area
Anyone in the UK who wants to use this kind of technology to prevent the birth of human babies with gene defects would have to seek approval from the Human Fertilisation and Embryology Authority.
A spokesman for the authority said: "There's a huge path to cross before that happens. No-one has ever made such an application."
At present, there would be stiff resistance from the scientific community if anyone wanted to do it.
It is more likely that the research could assist the development of transgenic animals - perhaps which have been designed to mimic human diseases so that cures can be tested.