Systems biology is an emergent field that aims at system-level understanding of biological systems. Since the days of Weiner, system-level understanding has been a long standing goal of biological sciences. Cybernetics, for example, aims at describing animals and machines from the control and communication theory.

Unfortunately, molecular biology has just started at that time, so that only phenomenological analysis has been possible. It was only recently that system-level analysis can be grounded on discoveries at molecular-level. With the progress of genome sequence project and range of other molecular biology project that accumulate in-depth knowledge of molecular nature of biological system, we are now at the stage to seriously look into possibility of system-level understanding solidly grounded on molecular-level understanding.

What does it mean to understand at "system level"? Unlike molecular biology which focus on molecules, such as sequence of nucleotide acids and proteins, systems biology focus on systems that are composed of molecular components. Although systems are composed of matters, the essence of system lies in dynamics and it cannot be described merely by enumerating components of the system. At the same time, it is misleading to believe that only system structure, such as network topologies, is important without paying sufficient attention to diversities and functionalities of components. Both structure of the system and components plays indispensable role forming symbiotic state of the system as a whole.

Within this context, (1) understanding of structure of the system, such as gene regulatory and biochemical networks, as well as physical structures, (2) understanding of dynamics of the system, both quantitative and qualitative analysis as well as construction of theory/model with powerful prediction capability, (3) understanding of control methods of the system, and (4) understanding of design methods of the system, are key milestones to judge how much we understand the system.

There are numbers of exciting and profound issues that are actively investigated, such as robustness of biological systems, network structures and dynamics, and applications to drug discovery. Systems biology is in its infancy, but this is the area that has to be explored and the area that we believe to be the main stream in biological sciences in this century.

H. Kitano, Systems Biology: a brief overview, Science, 295:1662-1664, 2002
H. Kitano, Computational Systems Biology, Nature, 420:206-210, 2002