W172 Nucleic Acids Research, 2009, Vol. 37, Web Server issue Figure 1. BioPortal resources tab displaying the ontology-indexed resources for the term Hepatocellular Carcinoma. Figure 2. BioPortal architecture. advantages in that they are light weight, simple and easier to integrate. BioPortal Web services include services to get ontology metadata, to get ontology content, to download an ontol- ogy, and to search ontologies. Each ontology in BioPortal is indexed with a stable ontology identiﬁer and is the same for all versions of the same ontology, ontology versions are indicated by a version identiﬁer, which changes from one version to the next. BioPortal Web services pro- vide a list of the latest versions of all BioPortal ontologies, enable callers to ﬁnd an ontology identiﬁer based on a version identiﬁer, ﬁnd all version identiﬁers for a speciﬁc ontology, and list ontology categories. Web services to get ontology content include services to get all root concepts,