been integrated to the Wikidata knowledge graph. This has led to the integration of OBO structured ontologies to data from other, non-OBO databases . For example, the integration of the Human Disease Ontology to Wikidata has enabled its link to the description of cell-lines from the resource Cellosaurus. One of the goals of the integration of OBO Foundry to Wikidata has been to lower the barriers for non-ontologists to contribute to and use ontologies. Wikidata is arguably easier to understand and use than the traditional ontology models (which require a high degree of specific expertise). Principles Summary of OBO Foundry Principles for development of an OBO-compatible life sciences ontology: Openness The ontologies are openly available and have to be released under either the license CC-BY 3.0 or under the public domain (CC0). The openness of the ontologies has enabled, for example, the import of terms from the Gene Ontology (one of the ontologies that follow OBO Principles) to the Wikidata project. Common format The ontologies have to be available in a common formal language. In practice, that means that ontologies that are part of the OBO foundry need to describe items unsing the formats OWL/OWL2 or OBO using a RDF/XML syntax to maximize interoperability. Orthogonality Terms should be unique in the OBO space, meaning that each item has a unique ontology prefix (such as CHEBI, GO, PRO) and a local numeric identifier within the ontology. The choice of a numerical ID was made in order to improve maintenance and evolution of the resources. In order to participate in OBO Foundry, ontologies have to be orthogonal and the concepts it models must be unique within OBO, so each concept has a single Uniform Resource Identifier (URI). New ontologies have, then, to reuse work done in other efforts. Despite the ideal of uniqueness of terms and interoperability, in practice, this is difficult to enforce, leading to the occurrence of term duplication. Furthermore, some ontologies do not reuse terms or even reuse terms inappropriately. Versioning Ontologies evolve in time, refining concepts and descriptions according to advances in the knowledge of their specific domains. In order to ensure that new versions are updated, but tools that use older version of the ontologies are still function, OBO enforces a system of versioning systems, with each ontology version receiving a unique identifier, either in the format of a date or a numbering system, and metadata dags.