ABSTRACT
The molecular interactions can be graphically represented by a network, where nodes of the network represent biomolecules and edges of the network represent molecular interactions. There are generally two kinds of molecular interactions: stimulation and inhibition. An inhibitory (stimulatory) edge is usually represented by a red (green) arrow. Alternatively, inhibition can be represented by the edge and stimulation can be represented by a usual edge →. The network description allows for a ready transfer of concepts and techniques that have developed in fields such as mathematics (graph theory), physics, sociology, and engineering. For example, concepts such as modularity and tolerances are well known in engineering, and may be useful to analyze biological networks as well. Indeed, it is widely agreed that biomolecular networks share at least two important structural properties with engineering systems: modularity and use of recurring circuit elements [1]. In the following, we give a brief introduction to the two properties, as well as some other structural properties.
