The is a relation forms the basic structure of GO. If we say A is a B, we mean that node A is a subtype of node B. For example, mitotic cell cycle is a cell cycle, or lyase activity is a catalytic activity. It should be noted that is a does not mean ‘is an instance of’. An ‘instance’, ontologically speaking, is a specific example of something; e.g. a cat is a mammal, but Garfield is an instance of a cat, rather than a subtype of cat. GO, like most ontologies, does not use instances, and the terms in GO represent a class of entities or phenomena, rather than specific manifestations thereof. However, if we know that cat is a mammal, we can say that every instance of cat is a mammal.