A gene product can be annotated to zero or more nodes of each ontology, at any level within each ontology; annotation of a gene product to one ontology is independent of its annotation to other ontologies. Annotations should reflect the normal function, process, or localization (component) of the gene product; an activity or location observed only in a mutant or disease state is therefore not usually included. The member databases of the GO Consortium use manual and automated methods to annotate genes or gene products using GO terms. Both manual and automated annotations are made according to two principles: first, every annotation must be attributed to a source, which may be a literature reference, another database or a computational analysis; second, the annotation must indicate what kind of evidence is found in the cited source to support the association between the gene product and the GO term. GO uses a simple controlled vocabulary to indicate the type of evidence found in the cited reference to support the annotation. See the GO annotation guide and evidence code documentation for more information.