What is annotation?

FAQ tags: 
What does it mean to do GO annotation of genes or proteins? Terms from the Gene Ontology are applied in the annotation of gene products or protein complexes in biological databases. GO annotations are associations made between gene products or protein complexes and the GO terms that describe them. Because a single gene may encode different products with very different attributes, GO recommends associating GO terms with database objects representing gene products rather than genes. If identifiers are not available to distinguish individual gene products, GO terms may be associated with an identifier for a gene; a gene object is associated with all GO terms applicable to any of its products. An annotation also includes an evidence code and a reference that supports the gene product/term association.