# Creating Reaction Rules

Reaction rules are the generators of species and reactions in a BioNetGen model. Each rule has four basic elements: reactant patterns, an arrow, product patterns, and a rate law. Reactant patterns are used to select a set of reactant species to which the rule will be applied. The arrow indicates whether the rule is applied in the forward direction only or in the forward and reverse directions. The product patterns define how the selected species are transformed by the rule and also act as the species selectors when the rule is applied in reverse if the reaction has a reverse direction. The default rate law is the elementary rate law, which gives the rate of a reaction as the product of a constant factor, the rate constant, and the reactant concentrations.

To create a reaction rule, species and molecules are placed as needed on both sides of a reaction rate on the Editor Window. Although there is no required order, simulation will proceed correctly only when the participating species and molecules have separately been registered within the Molecule Template Palette and the Species Palette. Once the reaction rule depiction is complete within the Editor Window it should be dragged and dropped into the Reaction Rule Palette.