The disease ontology adopted in TTD for both targets and drugs is the eleventh revision of International Classification of Diseases (ICD-11), which is developed and regularly updated by the World Health Organization (WHO). ICD-11 is the global standard for the organization, description and coding of information about diseases and causes of death. It consists of 17.000 categories, 80.000 concepts, 130.000 terms, >1.6 million clinical terms interpreted. It has 29 Chapters in total, which reflects major aspects of diseases. For example, Chapter 01 presents the certain infectious or parasitic diseases. The Chapter is numbered in Arabic numbers from 01-X. Categories have 4 characters and there are 2 additional levels of subcategories. The codes of the ICD–11 are alphanumeric and cover the range from 1A00.00 to ZZ9Z.ZZ.
Enzyme Classification Ontology
The enzyme classification ontology adopted in TTD for targets is the Enzyme Commission number (EC number). As a system of enzyme nomenclature, every EC number is associated with a recommended name for the corresponding enzyme-catalyzed reaction. It includes seven categories, which are oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, and translocases. Each group of enzymes in the above seven categories can be further divided into several subgroups according to the characteristics of the functional groups or bonds in the substrates. In order to show the properties of substrates or reactants more accurately, each subclass is further divided into subclasses and directly contains a quantity of enzymes.
Transporter Classification Ontology
The transporter classification ontology adopted in TTD for targets is the Transporter Classification Database (TCDB), which is an approved classification system for membrane transport proteins, including ion channels. Proteins are organized in a five level hierarchical system by TCDB, where the first two levels are the class and subclass, the second two are the family and subfamily, and the last one is the transport system.