Description: The human phenotype ontology (HPO) intends to offer a tool that will allow large-scale computational analysis of the human phenome. The HPO currently contains over 9500 terms, each of which describes an individual phenotypic anomaly. The terms are arranged in a directed acyclic graph and are connected by is-a edges, such that a term represents a more specific or limited instance of its parent term(s). All relationships in the HPO are is_a relationships, i.e. a simple class-subclass relationships. For instance, Abnormality of the feet is_a Abnormality of the lower limbs. The relationships are transitive, meaning that they are inherited up all paths to the root. Organ abnormality is the main subontology of the HPO and contains descriptions of clinical abnormalities. Additional subontologies are provided to describe inheritance patterns and onset/clinical course.