GO is generally organized as a tree-like hierarchy of functional terms, in which each term can have children that are more-specific classes of the parent class. As a matter of fact, the GO hierarchy is a DAG rather than a tree, as a GO term can have several parents. Indeed, because GO covers three domains of biology knowledge and relationships are defined only within each kind of knowledge, GO is in fact three non-overlapping DAGs determined by their namespaces: biological process, molecular function and cellular component.