The XML Schema for representing curated chemical-gene interactions in the Comparative Toxicongenomics Database (CTD; ctd.mdibl.org). A chemical-gene interaction. An interaction consists of two or more actors connected by one or more actions. An actor is either a chemical, a gene or a nested sub-interaction. An action is a physical, regulatory or biochemical interaction between actors. An interaction is reported in zero, one or more taxa (organisms), and is cited in one or more literature references. Taxa (organisms) in which the interaction occurs. Reference(s) in which the interaction is cited. Action(s) that occur among the actors in the interaction. Genes, chemicals and/or nested interactions that are actors in this interaction. A type of physical, regulatory or biochemical action in an interaction. The CTD code that represents the type of an action in this interaction. The CTD code that represents the degree to which the action type occurs: '+' (increases), '-' (decreases), '1' (affects) or '0' (does not affect). The integer that indicates the sequence in which this action appears relative to other actions in this interaction. The internal CTD ID of the action's parent element. A taxon in which the interaction occurs. The scientific name of a taxon (organism) in which the interaction occurs. The NCBI Taxonomy ID of a taxon (organism) in which the interaction occurs. A reference in which the interaction is cited. The accession ID of a reference in which the interaction is cited. The database name of a reference in which the interaction is cited. The action(s) that occur among the actors in the interaction. The genes, chemicals and/or nested interactions that are actors in this interaction. The type of the actor: 'chem', 'gene' or 'ixn' (nested interaction). The internal CTD ID of the actor. The internal CTD ID of the actor's parent element. The accession identifier of the actor. The integer indicating the sequence in which this actor occurs with respect to all actors in the interaction. A qualifier that indicates the specific type or aspect of the actor in the interaction. A qualifier that further indicates the specific type or aspect of the actor in the interaction.