Abstract
During last decade, ontologies have been successfully used as semantic guidelines while navigating through huge and ever increasing quantity of digital documents. Nevertheless, the size of most ontologies, especially those shared and accepted as standards in a given domain, tends to grow beyond the human capacity to grasp information. This growth is thus problematic for a lot of key applications that require user interactions. This paper proposes a method to identify and build a sub-ontology focused on user’s concepts of interest. Additional key concepts are identified using efficient algorithms that encode the calculus of two operators’ closures: least common ancestor and greatest common descendant. As an example, the resulting program, called OntoFocus (http://www.ontotoolkit.mines-ales.fr/) is used to restrict the large Gene Ontology to a subontology focused on terms used to annotate a gene related to breast cancer.