Simple method of modeling gene regulatory networks. Shown here is an oscillatory gene regulation network . The rate equations in this model assume that the transcription factor association and dissociation from the operator sites are at equilibrium. Under this assumption, the fractional saturation model  can be used to determine the rate of production of each gene product. The arrowheads are indicative of the type of reaction. For example, arrow from the genes to the proteins have a special arrowhead indicating that it is a transcription reaction. The mRNA step can be included automatically if needed from the context menu (mouse right-click).