The Gene Ontology (GO) is being developed with the goal of providing a set of structured vocabularies for the annotation of genes and their products. Since the publication of the original paper in Nature Genetics in 2000 [14], GO has become one of the most widely used and mature bio-ontologies, although it is still very much a work in progress. GO was designed to enable data integration, and allows genes to be classified according to their functional properties. A primary goal is to provide a single, explicit definition of each biological concept, which means that the terms can be applied consistently by all biologists and also that computational representations of biology have a consistent interpretation.