The Disease Ontology (DO) is a comprehensive knowledge base of 8043 inherited, developmental, and acquired human diseases. It serves as a semantic integration resource for disease and medical vocabularies, integrating concepts from MeSH, ICD, NCI's thesaurus, SNOMED CT, and OMIM. The DO is used for disease annotation by major biomedical databases and as a standard representation in biomedical ontologies. It also provides ontological cross-mappings between DO, MeSH, and OMIM. The DO project is incorporated into open-source tools to connect gene and disease data. The next iteration of the DO web browser will integrate extended relations and logical definitions. The DO was developed to create a unified structure for disease classification, enabling consistent use and application for annotating biomedical data. It addresses the complexity of disease nomenclature by including MeSH, OMIM, ICD, and SNOMED CT concepts. The DO web browser provides a framework for data mining, reasoning, and inference, enabling exploration of biomedical disease and gene data. The DO is an open-source ontological description of human disease, organized from a clinical perspective of disease etiology and location. The DO has become a community-driven, open, and extensible framework for capturing human disease knowledge through semantic relationships. It enables exploration of datasets and data resources through disease mappings available in clinical, gene, and genome study metadata. The DO's directed acyclic graph (DAG) presents terms linked by computable relationships in a hierarchy. The DO is organized into eight main nodes to represent various disease types. The DO continues to improve and expand the representation of all human disease with the addition of new terms. It provides ongoing documentation via the DO wiki, Facebook, LinkedIn, Twitter, and website. The DO provides an ontological definition of disease, enabling each type of disease to be singularly classified in a formalized structure. The DO has clarified its ontological scope with the adoption of the OGMS definition of disease. The DO's semantic integration includes broadening immune system, bone, mental, genetic, and infectious disease subtrees through collaborative efforts. The DO project provides an ontological framework for uniform data management and consistent annotation of human disease terms in biomedical databases and ontologies. DO terms and their DOIDs are used to annotate disease concepts in major biomedical resources. The DO's content and structure are logically organized into major types of disease to enable guided expansion of the ontology. The DO's stable HumanDO.obo file provides the basis for representing complex relationships between disease, disorder, and phenotype. The DO has begun to expand its set of cross-product relations linking DO terms to orthogonal ontologies. The DO links disease terminologies by cross-mapping and including concepts from standard clinical and medical terminologies into an ontological classification of disease. The DO identifies, integrates, and connects synonymous disease concepts in various terminologies. DO updates vocabulary mappings twice yearly from an extraction of term CUIsThe Disease Ontology (DO) is a comprehensive knowledge base of 8043 inherited, developmental, and acquired human diseases. It serves as a semantic integration resource for disease and medical vocabularies, integrating concepts from MeSH, ICD, NCI's thesaurus, SNOMED CT, and OMIM. The DO is used for disease annotation by major biomedical databases and as a standard representation in biomedical ontologies. It also provides ontological cross-mappings between DO, MeSH, and OMIM. The DO project is incorporated into open-source tools to connect gene and disease data. The next iteration of the DO web browser will integrate extended relations and logical definitions. The DO was developed to create a unified structure for disease classification, enabling consistent use and application for annotating biomedical data. It addresses the complexity of disease nomenclature by including MeSH, OMIM, ICD, and SNOMED CT concepts. The DO web browser provides a framework for data mining, reasoning, and inference, enabling exploration of biomedical disease and gene data. The DO is an open-source ontological description of human disease, organized from a clinical perspective of disease etiology and location. The DO has become a community-driven, open, and extensible framework for capturing human disease knowledge through semantic relationships. It enables exploration of datasets and data resources through disease mappings available in clinical, gene, and genome study metadata. The DO's directed acyclic graph (DAG) presents terms linked by computable relationships in a hierarchy. The DO is organized into eight main nodes to represent various disease types. The DO continues to improve and expand the representation of all human disease with the addition of new terms. It provides ongoing documentation via the DO wiki, Facebook, LinkedIn, Twitter, and website. The DO provides an ontological definition of disease, enabling each type of disease to be singularly classified in a formalized structure. The DO has clarified its ontological scope with the adoption of the OGMS definition of disease. The DO's semantic integration includes broadening immune system, bone, mental, genetic, and infectious disease subtrees through collaborative efforts. The DO project provides an ontological framework for uniform data management and consistent annotation of human disease terms in biomedical databases and ontologies. DO terms and their DOIDs are used to annotate disease concepts in major biomedical resources. The DO's content and structure are logically organized into major types of disease to enable guided expansion of the ontology. The DO's stable HumanDO.obo file provides the basis for representing complex relationships between disease, disorder, and phenotype. The DO has begun to expand its set of cross-product relations linking DO terms to orthogonal ontologies. The DO links disease terminologies by cross-mapping and including concepts from standard clinical and medical terminologies into an ontological classification of disease. The DO identifies, integrates, and connects synonymous disease concepts in various terminologies. DO updates vocabulary mappings twice yearly from an extraction of term CUIs