All ontologies in the OBO library have a permanent URL (PURL) that always resolves to the OWL for the ontology (most ontologies have a PURL that resolves to an OBO Format version too). For example:


Additionally, some ontologies may have additional artefacts/products such as slims available within their namespace.

These purls should always be used to reference the ontology. Maintainers of ontologies have the option of deciding what their PURL resolves to. Many choose to resolve to the central OBO build URLs.

Historically these have always resolved to – we have recently changed this to

Please note this should not affect you if you are a consumer of ontologies - you should always use the PURL directly.

(although hopefully ontologies should now resolve faster, as we are using Amazon S3 plus CloudFront).

If you are a providers of ontologies and you have made use of the central OBO build, then your PURLs have been switched to use the new scheme as part of this pull request.