As we look back at 2007, it's time to judge the best books, the best movies—and the best scientific discoveries. The prestigious journal Science came out with its "Breakthrough of the Year" in the December 20th edition. The runner-up was the discovery that skin cells could be manipulated to behave like embryonic stem cells. The number one pick? Human genetic variation. Scientists have been learning increasing amounts about how genes influence traits and diseases. At the same time, we hear about how little difference there is between us and our not-so-distant cousins, chimps and gorillas. But this year, researchers sequenced the genomes of thousands of individual people. They looked at variation in short stretches of DNA called single nucleotide polymorphisms. Scientists discovered how much we vary from one another on this basic level—crucial info for understanding everything from who gets diabetes to who's at risk for breast cancer to maybe even why I can't stand cilantro. For more information on the breakthroughs of the year, go to sciencemag.org.