The Gene Expression Database (GXD) is a community resource of gene expression information for the laboratory mouse. The database is designed as an open-ended system that can integrate different types of expression data. New expression data are made available on a daily basis. Thus, GXD provides increasingly complete information about what transcripts and proteins are produced by what genes; where, when and in what amounts these gene products are expressed; and how their expression varies in different mouse strains and mutants. GXD is integrated with the Mouse Genome Database (MGD). Continuously refined interconnections with sequence databases and with databases from other species place the gene expression information in the larger biological and analytical context. GXD is accessible through the Mouse Genome Informatics Web site at http://www.informatics.jax.org/ or directly at http://www.informatics.jax.org/menus/expression_menu.shtm l