The Mouse Genome Informatics (MGI, http://www.informatics.jax.org/) resource provides the research community with access to information on the genetics, genomics, and biology of the laboratory mouse. Core data in MGI include gene characterization and function, phenotype and disease model descriptions, DNA and protein sequence data, gene expression data, mammalian orthologies, polymorphisms, mapping data, and links to other bioinformatics databases. Semantic integration is supported through the use of standardized nomenclature and controlled vocabularies such as the mouse Anatomical Dictionary and the Gene Ontologies. MouseBLAST offers a fast, sequence-based method for identifying genes and searching for similarity, and provides links from sequence to MGI biological data. MGI extracts and organizes data from primary literature. MGI data are shared with and widely displayed by other bioinformatics resources. The database is updated nightly with curated annotations and regularly adds new datasets and enhances user interface features. This unit provides a guide to using the MGI bioinformatics resource.