The Gene Expression Database (GXD) is an easily searchable, freely available database of mouse developmental gene expression information (www.informatics.jax.org/expression.shtml). It provides researchers with critical insights into the function of genes and the molecular mechanisms of development, differentiation, and disease processes. GXD captures expression data from wild type and mutant mice. It integrates data from different assay types, including RNA in situ hybridization, immunohistochemistry, knock-in in situ reporter, RT-PCR, as well as Northern and Western blot experiments. Data are acquired from the literature and from groups engaged in large-scale expression studies. Currently GXD contains over 1.5 million expression results for over 14,000 genes. GXD contains detailed metadata recorded in standardized ways, including: time and tissue of expression (or non-expression); pattern and strength of expression; numbers and sizes of detected bands (for blots); genetic background of the samples; and a description of the probe/antibody used in the assay. These data are accompanied by over 275,000 images, allowing users to view the primary data and interpret it themselves. As an integral part of the Mouse Genome Informatics (MGI) resource, GXD integrates its expression data with other genetic, functional, phenotypic, and disease-oriented data, thus allowing researchers to evaluate expression data in the larger context; search by a wide variety of biologically- and biomedically-relevant parameters; and discover new data connections to help in the design of new experiments. Recently developed features provide quick and intuitive access to the data. These include: interactive tissue-by-developmental stage and tissue-by-gene matrix views; image summaries defined by user search criteria; and data filters that can be used to iteratively refine search results. The expression section of MGI gene detail pages now features a graphical gene expression overview, as well as links to expression data from chicken, Xenopus, and zebrafish, enabling a gene-based comparison of expression information between mouse and these species. GXD is funded by NIH grant HD062499.