Fig. 1. A section of the Gene Ontology (GO) resource (Gene Ontology Consortium, 2000, www.geneontology.org/), showing the pathways that represent the process of DNA replication as part of a hierarchical tree. Each term is assigned a unique GO identifier. The plus and minus signs indicate that the corresponding terms can be, or have been, expanded to display parent–child relationships. The circled P and I symbols indicate ‘part of’ or ‘is a’ relationships between a particular term and its parent or children. Numbers enclosed in parentheses show the gene associations annotated to this term or to a more specific term below this in the GO tree. The ontology establishes precise, defined relationships between the terms that can be used to implement queries that are much more complex than those possible with simple keywords. As an illustration, this section of the ontology reveals that the process of DNA replication is found as part of three different pathways—cell cycle, mitotic S phase and DNS metabolism—the terms of all of which can be used to recover information about gene products associated with this process. Adapted from Ashburner et al. (2001).