A DNA breakthrough could lead to a contraceptive pill for men.
Scientists found a gene that is vital for male fertility in humans and almost all other mammals.
Blocking it with a drug could cripple sperm and stop unwanted pregnancies.
But stopping the medication would allow the gene to switch back on and make the patient fertile again.
Lab experiments on mice found switching off gene Arrdc5 slashed sperm production by 28 per cent and 98 per cent of those still produced were damaged.
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Professor Jon Oatley, from Washington State University, said: “Our study identifies this gene for the first time as being expressed only in testicular tissue, nowhere else in the body.
“When it is inactivated in males, they make sperm that cannot fertilise an egg, and that’s a prime target for male contraceptive development.
“You don't want to wipe out the ability to ever make sperm – just stop the sperm that are being made from being made correctly.
“Then, in theory, you could remove the drug and the sperm would start being built normally again.”
Writing in the journal Nature Communications, Prof Oatley said his team first found the gene in the testicles of mice, pigs, cows and humans.
It is used to produce a specific protein that is vital for making healthy sperm.
Without the protein, the sperm are made with abnormal heads and swim very slowly, meaning they do not work.
There are currently no medical contraceptives for men, while women have a range of hormone options – but many suffer side effects.
Scientists are trying to invent a drug for blokes so couples have more choice.
A trial by the National Institutes of Health in the US is showing promising results for the Nestorone hormone gel, which slashes sperm counts after being rubbed into the skin.
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Prof Oatley and his team are now working on a drug for humans and have filed a patent for a contraceptive blocking the protein made by this gene.
Prof Oatley added: “We don't really have anything on the male side for contraception other than surgery and only a small percentage of men choose vasectomies.
“If we can develop this discovery into a solution for contraception, it could have far-ranging impacts.”