SCIENTISTS AT THE SCRIPPS RESEARCH INSTITUTE IN LA JOLLA, Calif., say they have a nifty way to lower populations of rodents that cause crop damage and diseases: birth control. Molecular biologist Jeffrey Bleil and his colleagues have identified the protein receptor on mice sperm that permits them to recognize and to attach to the female egg. When male mice are fed synthetic copies of this protein, called sp56, the protein prods the animals' immune systems to mount an attack against it--and against the sperm cells that bear the genuine receptor.

One dose of the protein seems to render both male and female mice sterile for weeks to months at a stretch. It also works for rats and hamsters, which have the sp56 protein on their sperm. Humans lack this protein. So, even if a person accidentally ingested it, the protein would have no effect. But Bleil thinks human sperm may have some equivalent receptor. Its identification could lead to a new form of contraception. In mice, at least, there seem to be no side effects.

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