Examples of active elementary modes. (a) This figure shows the citrate cycle map from KEGG. Enzymes colored in red are coded by genes induced during the stationary phase. They correspond exactly to elementary mode number 36 of the citrate cycle, with the exception of one enzyme in yellow (22.214.171.124). (b) The sulfur metabolism map from KEGG. Enzymes colored in red are coded by genes found induced when yeast is exposed to tetrachloro-isophthalonitrile. These enzymes compose the entire elementary mode number 3 with the exception of two of them (in yellow): YGR012W is not induced but YLR303W is induced and fulfils the same function (EC 126.96.36.199); in the second case, two enzymes can fulfill the same function, so even if one is missing, the other completes the metabolic route (EC 188.8.131.52 and EC 184.108.40.206). Enzymes in grey are present in S. cerevisiae but do not belong to the elementary mode.