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Men make a thousand sperm every second and to have an effective contraceptive,
每秒中，男性可生产出一千个精子，所以有效的避孕药
you need to get that level of sperm production down to one percent of its normal value.
必须把这个精子生产率降到正常值的1%。
The good news is, this is possible, almost.
好消息是，这是可以做到的，几乎可以。
The most studied approach has been to use hormones to suppress sperm production.
被研究最多的方法是用激素抑制精子生产。
Testosterone and progesterone, when administered together,
睾丸酮和黄体酮，当一起被服用的时候，
will suppress the signals from the brain to the testes to make sperm,
会抑制大脑发射的让睾丸生产精子的信号，
and in about 90 percent of men, sperm production after three to four months will stop.
在90%左右的男性中，这个方法能使精子的产生在三四个月后停止。
Unfortunately, 10 percent of men don't respond to these hormonal regimens for reasons that aren't understood.
不过很遗憾，还有10%的男性不会对这些激素疗法做出反应，原因我们目前还不了解。
For the last several years, my colleagues and I have been taking a different approach to male contraceptive development,
过去几年里，我和我的同事采用了另一种解决男性避孕问题的方法，
one that doesn't involve the administration of hormones.
一个不需要激素疗程的方法。
Specifically, we are looking to block the function of vitamin A in the testes.
具体来说，我们在研究如何阻止睾丸中维生素A的活动。
Why? Well, for over 90 years it's been known that you need vitamin A to make sperm.
为什么这么做呢？90多年来，我们一直都知道你需要维生素A来生产精子。
Animals who are deprived of vitamin A in their diet stop making sperm
饮食中缺乏维生素A的动物会停止生产精子，
and restart making sperm again when the vitamin A is reintroduced.
当维生素A被重新引进饮食后，它们又会重新开始精子的生产。
The vitamin A that we ingest is converted by a family of enzymes to something called retinoic acid.
我们摄入的维生素A会被一个类别的酶转化成视黄酸。
One of these enzymes is found only in the testes.
其中一种酶只在睾丸才有。
It's this enzyme that we are attempting to block.
我们试图限制的就是这种酶。
The blockade of this enzyme should deprive the testes of retinoic acid
阻断这个酶应该可以使精子丧失视黄酸，
and stop sperm production without affecting vitamin A's functions elsewhere in the body
从而停止精子生产，同时并不影响维生素A在身体其他部分的功能。
We're testing this approach in animals and hope to move to human testing soon.
我们正在动物体内测试这个方法，希望不久后可以在人体内测试。
