Gene Ontology http://www.geneontology.org

Controlled vocabulary ("GO terms") to describe genes and gene products.

Three hierarchies of terms (directed acyclic graphs).

• Molecular function
  Tasks performed by individual gene products; chemistry

• Biological process
  Broad biological goals accomplished by ordered assemblies of molecular functions

• Cellular component
  Subcellular structures, locations, macromolecular complexes

Association: Connect GO terms with a gene or gene product, based on evidence (references) of some kind.

Produced by curators at different database sites (SGD, TAIR, Flybase, MGI, Wormbase,... but not Ensembl!).