Abstract Motivation: Molecular biotechnology now makes it possible to build elaborate systems models, but the systems biology community needs information standards if models are to be shared, evaluated and developed cooperatively. Results: We summarize the Systems Biology Markup Language (SBML) Level 1, a free, open, XML-based format for representing biochemical reaction networks. SBML is a software-independent language for describing models common to research in many areas of computational biology, including cell signaling pathways, metabolic pathways, gene regulation, and others. Availability: The specification of SBML Level 1 is freely available from http://www.sbml.org/ Contact: sysbio-team@caltech.edu * To whom correspondence should be addressed.

The systems biology markup language (SBML): a medium for representation and exchange of biochemical network models