which the predictive model is used to identify the best experiments to reach specific goals. To understand complex systems, the iterative cycle of model- driven experiments and experimental data-driven modeling allows system- atic analysis of the underlying princi- ples and logic of complex biomedical systems. However, Dr. Van Driel also claims that when one asks 3 persons to define sys- tems biology, one gets 5 answers. June 2009 No. 4 Principle of systems biology