Serotonin regulates sexual behavior in both mice and men. "Neuroscientist Yi Rao of Peking University and the National Institute of Biological Sciences in Beijing, and his collaborators have now shown that serotonin also underlies a male's decision to woo a female or another male. They published their results in the March 24 issue of the journal Nature. Rao and his team genetically engineered male mice to lack either serotonin-producing neurons or a protein that is crucial for making serotonin in the brain. Both types of altered mouse couldn't make serotonin."