ArrayExpress is a public repository for microarray gene expression data established at the European Bioinformatics Institute (EBI) in 2002. It supports the MIAME (Minimum Information About a Microarray Experiment) standards and stores well-annotated raw and normalized data. As of November 2004, ArrayExpress contains data from approximately 12,000 hybridizations across 35 species. Data can be submitted online or via local databases/LIMS in a standard format, with password-protected access for pre-publication data. The repository provides a query interface to retrieve data by accession number or through various parameters such as species, author, and array platform. A curated subset of normalized data is available in a data warehouse, which can be queried by gene and sample properties. Data visualization and analysis are facilitated using Expression Profiler. The ArrayExpress suite includes MIAMExpress, a web-based data submission tool, the main repository, the curated data warehouse, and Expression Profiler. The repository has seen rapid growth, with a 10-fold increase in content over the last 12 months. Future developments include extending MIAMExpress for large-scale submissions, adding a graph-based visualization tool, unifying the repository and data warehouse interfaces, and integrating ArrayExpress into EBI services.ArrayExpress is a public repository for microarray gene expression data established at the European Bioinformatics Institute (EBI) in 2002. It supports the MIAME (Minimum Information About a Microarray Experiment) standards and stores well-annotated raw and normalized data. As of November 2004, ArrayExpress contains data from approximately 12,000 hybridizations across 35 species. Data can be submitted online or via local databases/LIMS in a standard format, with password-protected access for pre-publication data. The repository provides a query interface to retrieve data by accession number or through various parameters such as species, author, and array platform. A curated subset of normalized data is available in a data warehouse, which can be queried by gene and sample properties. Data visualization and analysis are facilitated using Expression Profiler. The ArrayExpress suite includes MIAMExpress, a web-based data submission tool, the main repository, the curated data warehouse, and Expression Profiler. The repository has seen rapid growth, with a 10-fold increase in content over the last 12 months. Future developments include extending MIAMExpress for large-scale submissions, adding a graph-based visualization tool, unifying the repository and data warehouse interfaces, and integrating ArrayExpress into EBI services.