What's hot in biology right now? Well, an educated guess by even a casual observer would probably be pretty accurate. All you have to do is read the paper or watch the news. But, why guess? Check out Sciencewatch, a site hosted by Thomson Reuters which tracks trends in the sciences. They estimate the impact of a research paper by looking at the number of times it's cited in someone else's work. Take a look at the hot papers for 2009, and you'll see that they're dominated by stem cell work, human genome investigations, and AIDS research. The Scientist, an online life sciences magazine, tells us that the most cited paper in biology over the last couple of years was the work of Kazutoshi Takahashi and his colleagues, most of them in the Department of Stem Cell Biology at Kyoto University in Japan. It showed the pluripotent stem cells could be produced from adult human fibroblasts, and was cited over 500 times in 2009. If you're into stem cell research, check it our here.