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David Clapham, a physician at Harvard Medical School, was studying calcium ion channels—chemical triggers that switch electrical activity on or off in cells—when he noticed an unusual type of channel that exists only on the tails of sperm. Turn it off, he reasoned, and the sperm would stop moving. Could this be the long-sought male contraceptive?

As a test, Clapham and his Harvard collaborators created genetically altered male mice that do not produce this sperm channel and then placed the animals in cages with female mice. "They behaved exactly like normal males, but they were infertile," he says. Further studies under the microscope showed the channel-free sperm move only one-third as quickly as normal sperm and lack both direction and follow-through. "Normally when a sperm approaches the egg, it goes into a frenzied state of hyperactivity that allows it to burrow through the tough outer covering," Clapham says. The genetically altered sperm lacked the energy to make that final lunge. He does not propose breeding infertile people, of course. Rather, he plans to work with pharmaceutical companies to find a drug that temporarily blocks the calcium ion channel of human sperm. "Ideally, it would be close to 100 percent effective, and because it's expressed only on sperm, there would be no side effects."

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