Once the draft network is ready, a mathematical formulation of the network is made through forming a stoichiometric matrix (S). In the matrix (Fig. 10.3b), rows cor­respond to metabolites, and columns assign reactions. The entries in the matrix correspond to the stoichiometric coefficients of each individual metabolite that contributes to each reaction. A negative coefficient indicates the consumption of a metabolite in a reaction. A positive coefficient indicates the production of a metabolite in a reaction. A zero coefficient is for metabolites that have no contri­bution to the reaction.