The BridgeDb App (previously known as CyThesaurus in Cytoscape 2.8) is a Cytoscape app that performs identifier mapping services based on various resources. BridgeDb provides utilities for mapping attributes of source ID types to new attributes of destination ID types. Currently, the application supports ID mapping resources from delimited text, BridgeDb file, BrigeDb web service, and BioMart web service. This app utilizes the [http://bridgedb.org/ BridgeDb API]. The BridgeDb project also provides the relational database content for WikiPathways, Pathvisio, GenMAPP-CS, and many other bioinformatics tools, which is in turn primarily based on content and relationships extracted from Ensembl database resources.