Deterministic and stochastic mathematical models of macromolecular processes along regulatory networks provide great insights on the physiological properties of cells. An appropriate way to deal with the challenges of mathematical modeling is to decompose the networks into smaller components. However, most available software does not support any notion of composition.

The authors address this issue by suggesting a formal approach for model composition, and present relevant extensions to the Systems Biology Markup Language (SBML). Their approach is then applied to a model for the eukaryotic cell cycle engine in order to show its appropriateness for biochemical systems.

The paper’s major focus is on the implementation aspects of model composition (the mathematical difficulties are disregarded). Because of this, the authors’ approach will interest bioinformatics researchers more than mathematical biologists. Although this work does not fulfill all of the reader’s expectations (raised by its title), it still carefully discusses the problem of model composition. Thus, I recommend it for readers interested in the modeling and simulation of biochemical processes on networks.