Relationships between database nodes in NCBI Entrez. Each node represents an Entrez database. Entrez databases group data objects of the same type, e.g. published articles in PubMed or nucleotide sequences of DNA/RNA in the Nucleotide database. Links between nodes reflect possible relationships between data, e.g. given an annotated nucleotide sequence we can quickly find the protein that is the product of its translation, and how we can traverse from one database to another discovering more about the particular object. Figure reproduced from [Ostell J., 2002].