A contraceptive pill that could be taken by men as well as women has come a step closer to being developed.
A breakthrough in the understanding of the biology of sperm could lead to a unisex version of the Pill, US government-funded scientists said last night.
They have identified a protein in sperm that gives it the energy it needs to power towards the female egg and break its way into it.
As the researchers put it, the ABHD2 protein allows the sperm's tail to "crack like a whip" and "power kick" its way into the egg.
Dr Stuart Moss, of the US National Institutes of Health, said: "Developing new compounds that block ABHD2 ultimately may yield new contraceptive methods that prevent sperm from reaching the egg."
Melissa Miller, a researcher at the University of California, Berkeley, added: "What is really cool is that we have an actual target for unisex contraceptive development."
Theoretically, such a Pill could be taken by either sex, allowing men to take a greater share of the burden of contraception.
Conversely, a drug that enhances the protein may help infertile couples to have children of their own, the journal Science reports.
While women have had access to oral contraceptives for more than half a century, the quest for a male Pill has been thwarted by biology. Whereas a woman normally releases one egg a month, a healthy man makes 1,000 sperm with every heartbeat.
Professor Mike Wyllie, an expert in drug development and male sexual health, said: "This study has allowed understanding of the control of male fertility to leapfrog, or at least equal, our knowledge of the female reproductive system."