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Molecular approaches to understanding the functional circuitry of the nervous system promise new insights into the relationship between genes, brain and behaviour. The cellular diversity of the brain necessitates a cellular resolution approach towards understanding the functional genomics of the nervous system. We describe here an anatomically comprehensive digital atlas containing the expression patterns of ∼20,000 genes in the adult mouse brain. Data were generated using automated high-throughput procedures for in situ hybridization and data acquisition, and are publicly accessible online. Newly developed image-based informatics tools allow global genome-scale structural analysis and cross-correlation, as well as identification of regionally enriched genes. Unbiased fine-resolution analysis has identified highly specific cellular markers as well as extensive evidence of cellular heterogeneity not evident in classical neuroanatomical atlases. This highly standardized atlas provides an open, primary data resource for a wide variety of further studies concerning brain organization and function. Brain bank A new frontier has been reached in both neuroscience and genetics. The expression of each of the roughly 22,000 genes of the mouse genome has been mapped, at cellular resolution, across all major structures of the mouse brain. This achievement is part of the Allen Brain Atlas project. Lein et al . describe the development of the atlas (freely available on http://www.brain-map.org ) and report gene expression patterns that both support and challenge established views of brain anatomy. The atlas includes in situ images and 'heat maps' of signal intensity for each gene and brain region on a colorimetric scale. Despite predictions that the brain would express a limited number of genes, about 80% of all mouse genes are expressed; 70% of gene signals localize to fewer than 20% of all brain cells, suggesting that most localize to small brain regions. Cover image: Chris Lau, Allen Institute for Brain Science. The expression of each of the roughly 22,000 genes of the mouse genome has been mapped, at cellular resolution, across all major structures of the mouse brain, revealing that 80% of all genes appear to be expressed in the brain.

Considerable progress has been made in understanding variations in gene sequence and expression level associated with phenotype, yet how genetic diversity translates into complex phenotypic differences remains poorly understood. Here, we examine the relationship between genetic background and spatial patterns of gene expression across seven strains of mice, providing the most extensive cellular-resolution comparative analysis of gene expression in the mammalian brain to date. Using comprehensive brain-wide anatomic coverage (more than 200 brain regions), we applied in situ hybridization to analyze the spatial expression patterns of 49 genes encoding well-known pharmaceutical drug targets. Remarkably, over 50% of the genes examined showed interstrain expression variation. In addition, the variability was nonuniformly distributed across strain and neuroanatomic region, suggesting certain organizing principles. First, the degree of expression variance among strains mirrors genealogic relationships. Second, expression pattern differences were concentrated in higher-order brain regions such as the cortex and hippocampus. Divergence in gene expression patterns across the brain could contribute significantly to variations in behavior and responses to neuroactive drugs in laboratory mouse strains and may help to explain individual differences in human responsiveness to neuroactive drugs.