The Allen Institute for Brain Science in Seattle has completed a three-dimensional, Web-based atlas of the mouse brain that details the expression of its 21,000 genes. The enormous database at can be freely accessed by the public. A sample image (above) shows the presence of a gene, Emp 1, in the left hemisphere. Each yellow sphere represents expression of the gene; larger spheres correspond to greater expression density. The institute notes that 90 percent of mouse genes have a direct human counterpart and that the data could therefore provide insights into how the human brain works and how illnesses such as Alzheimer’s and epilepsy compromise it. Scientists plan to start compiling a similar database for the human cortex, which is central to higher-order thinking.