The National Center for Biotechnology Information (NCBI) provides a wide range of database resources and tools for the analysis and retrieval of biological data. These resources include Entrez, PubMed, PubMed Central, LocusLink, the NCBI Taxonomy Browser, BLAST, BLAST Link (BLink), Electronic PCR, OrfFinder, Spidey, RefSeq, UniGene, HomoloGene, ProtEST, dbMHC, dbSNP, Cancer Chromosome Aberration Project (CCAP), Entrez Genomes, the Map Viewer, Model Maker, Evidence Viewer, Clusters of Orthologous Groups (COGs) database, Retroviral Genotyping Tools, SARS Coronavirus Resource, SAGEmap, Gene Expression Omnibus (GEO), Online Mendelian Inheritance in Man (OMIM), the Molecular Modeling Database (MMDB), the Conserved Domain Database (CDD), and the Conserved Domain Architecture Retrieval Tool (CDART). These resources are accessible through the NCBI home page at http://www.ncbi.nlm.nih.gov. Entrez is an integrated database retrieval system that allows text searching of a diverse set of databases, including DNA and protein sequences, the NCBI taxonomy, genomes, gene expression data, and the biomedical literature. PubMed includes over 12.7 million references and abstracts in MEDLINE, with links to the full text of more than 4000 journals. PubMed Central (PMC) is a digital archive of peer-reviewed journals in the life sciences, with over 130 journals depositing full text articles. The NCBI taxonomy database indexes over 150,000 organisms, and the Taxonomy Browser can be used to view the taxonomic position or retrieve data from any of the principal Entrez databases for a particular organism or group. LocusLink provides an interface to curated sequences and descriptive information about genes, with links to NCBI's Map Viewer, Evidence Viewer, Model Maker, BLAST Link, and other gene-related resources. The BLAST family of sequence-similarity search programs, including BLAST, MegaBLAST, and Discontiguous MegaBLAST, are used for sequence comparisons and alignments. The BLAST Link (BLink) displays pre-computed protein BLAST alignments for each protein sequence in the Entrez databases. Resources for gene-level sequences include UniGene, which automatically partitions GenBank sequences into non-redundant gene-oriented clusters, and ProtEST, which presents pre-computed BLAST alignments between protein sequences from model organisms and the six-frame translations of UniGene nucleotide sequences. Resources for genome-scale analysis include Entrez Genomes, which provides access to genomic data contributed by the scientific community, and the Map Viewer, which displays genome assemblies using sets of aligned chromosomal maps. The Model Maker is used to construct transcript models using combinations of putative exons derived from ab initio predictions or from the alignment of GenBankThe National Center for Biotechnology Information (NCBI) provides a wide range of database resources and tools for the analysis and retrieval of biological data. These resources include Entrez, PubMed, PubMed Central, LocusLink, the NCBI Taxonomy Browser, BLAST, BLAST Link (BLink), Electronic PCR, OrfFinder, Spidey, RefSeq, UniGene, HomoloGene, ProtEST, dbMHC, dbSNP, Cancer Chromosome Aberration Project (CCAP), Entrez Genomes, the Map Viewer, Model Maker, Evidence Viewer, Clusters of Orthologous Groups (COGs) database, Retroviral Genotyping Tools, SARS Coronavirus Resource, SAGEmap, Gene Expression Omnibus (GEO), Online Mendelian Inheritance in Man (OMIM), the Molecular Modeling Database (MMDB), the Conserved Domain Database (CDD), and the Conserved Domain Architecture Retrieval Tool (CDART). These resources are accessible through the NCBI home page at http://www.ncbi.nlm.nih.gov. Entrez is an integrated database retrieval system that allows text searching of a diverse set of databases, including DNA and protein sequences, the NCBI taxonomy, genomes, gene expression data, and the biomedical literature. PubMed includes over 12.7 million references and abstracts in MEDLINE, with links to the full text of more than 4000 journals. PubMed Central (PMC) is a digital archive of peer-reviewed journals in the life sciences, with over 130 journals depositing full text articles. The NCBI taxonomy database indexes over 150,000 organisms, and the Taxonomy Browser can be used to view the taxonomic position or retrieve data from any of the principal Entrez databases for a particular organism or group. LocusLink provides an interface to curated sequences and descriptive information about genes, with links to NCBI's Map Viewer, Evidence Viewer, Model Maker, BLAST Link, and other gene-related resources. The BLAST family of sequence-similarity search programs, including BLAST, MegaBLAST, and Discontiguous MegaBLAST, are used for sequence comparisons and alignments. The BLAST Link (BLink) displays pre-computed protein BLAST alignments for each protein sequence in the Entrez databases. Resources for gene-level sequences include UniGene, which automatically partitions GenBank sequences into non-redundant gene-oriented clusters, and ProtEST, which presents pre-computed BLAST alignments between protein sequences from model organisms and the six-frame translations of UniGene nucleotide sequences. Resources for genome-scale analysis include Entrez Genomes, which provides access to genomic data contributed by the scientific community, and the Map Viewer, which displays genome assemblies using sets of aligned chromosomal maps. The Model Maker is used to construct transcript models using combinations of putative exons derived from ab initio predictions or from the alignment of GenBank