The Pathway Interaction Database (PID) is a free biomedical database of human cellular signaling pathways. The database contains information about the molecular interactions and reactions that take place in cells, with a particular focus on processes that might be relevant to cancer research and treatment. The database was established as collaboration between the U.S. National Cancer Institute, NIH and Nature Publishing Group in 2005 and was launched in November 2006. In September 2012, active curation was stopped and the PID data are now available in the [http://www.ndexbio.org/#/user/301a91c6-a37b-11e4-bda0-000c29202374 Network Data Exchange, NDEx].