MK4MDD aims to offer research community a comprehensive resource and analysis platform for MDD research. The keywords for paper search is described in "Literature search and data extraction" part of Introduction. It is really a challenge to cover all relevant terms of such a complex human disorder with broad clinical manifestations and multiple risk factors. Besides expanding coverage of MK4MDD on the basis of current version, we welcome users to propose new search keywords or terms and suggest papers to help us maintain the database. We will evaluate the keywords and papers according to our criteria on data collection and add corresponding components and relationships into MK4MDD if they are qualified.