When I tell people that I'm trying to develop a contraceptive pill for men, the response usually varies along gender lines. Women say something like, "Fantastic. It's about time. When?" (Laughter) Men have one of two responses. They either love the idea, or they look at me a little warily and wonder what exactly I have in store for their testicles. (Laughter) So why does the world need a male pill? Well, what if I told you that of the six million pregnancies annually in the United States, three million of them are unintended? That's half. That's a really surprising number. And those three million unintended pregnancies account for the vast majority of the more than one million abortions annually performed in the United States. Happily, the rate of unintended pregnancy has fallen in the last few years by about 10 percent. This is because more women are using effective, long-acting, reversible forms of contraception. But we still have a long way to go. One approach that's finally becoming a real possibility is better contraceptive options for men. Think about it. We have over a dozen methods of contraception for women: pills, patches, IUDs, shots, sponges, rings, etc. For men, we've had the same two options for more than a hundred years: condoms and vasectomy. Despite having only two options, both of which have significant drawbacks, men currently account for 30 percent of all contraceptive use, with 10 percent of couples relying on vasectomy and 20 percent of couples using condoms. Why are 20 percent of couples relying on condoms for contraception when condoms have a one-year failure rate of over 15 percent? It's because many women can't either safely take currently available female contraceptives, for reasons such as blood clots, or they can't tolerate the side effects. So if we think a male contraceptive would be useful, the next question is: How do we go about developing one? Well, there's two general approaches. The first approach is to try and interfere with the way the sperm swim towards or bind to the egg. This approach turns out to be really difficult, because it's hard to get enough medication in the small volume of the ejaculate and have it still work inside the female reproductive tract. This is why there's been a lot more work done on the second approach, which is turning off sperm production entirely. This is also challenging. Why? Turns out that men make a lot of sperm. (Laughter) Men make a thousand sperm every second and to have an effective contraceptive, you need to get that level of sperm production down to one percent of its normal value. The good news is, this is possible, almost. The most studied approach has been to use hormones to suppress sperm production. Testosterone and progesterone, when administered together, will suppress the signals from the brain to the testes to make sperm, and in about 90 percent of men, sperm production after three to four months will stop. Unfortunately, 10 percent of men don't respond to these hormonal regimens for reasons that aren't understood. For the last several years, my colleagues and I have been taking a different approach to male contraceptive development, one that doesn't involve the administration of hormones. Specifically, we are looking to block the function of vitamin A in the testes. Why? Well, for over 90 years it's been known that you need vitamin A to make sperm. Animals who are deprived of vitamin A in their diet stop making sperm and restart making sperm again when the vitamin A is reintroduced. The vitamin A that we ingest is converted by a family of enzymes to something called retinoic acid. One of these enzymes is found only in the testes. It's this enzyme that we are attempting to block. The blockade of this enzyme should deprive the testes of retinoic acid and stop sperm production without affecting vitamin A's functions elsewhere in the body. We're testing this approach in animals and hope to move to human testing soon. Obviously, the impact of such a male contraceptive would go well beyond reproductive biology. It's interesting to speculate about the effect that it would have on relationships between men and women. One intriguing possibility is that a man could monitor his contraceptive status over time. In the last several years, two groups have introduced home sperm-testing devices that are iPhone-based and that are easy to use. A man could test his sperm count and share the result with his partner. If the man's sperm count were zero, the man and his partner would feel very comfortable relying on his contraceptive. A tool like this, coupled with a male contraceptive, could greatly increase the role for men in preventing unintended pregnancy. The researchers who are working on male contraception are trying to create a better future for couples, a future where contraception is no longer considered just "a woman's issue," rather an issue for couples to decide together. So why does the world need a male pill? Well, I believe that a male pill will help reduce the stubbornly high rates of unintended pregnancy and abortion and allow men to equally participate in contraception. Thank you. (Applause)