Example of the GX-FBA approach. a) An abbreviated model of central carbon metabolism with glucose (GLU) as the sole nutrient source with a fixed ATP maintenance cost of 12. FBA predicted fluxes in blue, flux constraints in red. b) A 2D schematic of possible FBA flux regions for reactions 2 and 5. The feasible flux region is shaded yellow, and the maximal growth solution is marked “FBA solution.” Non-growth associated maintenance cost implies that the flux through the glycolytic reactions (1 & 2) cannot be zero. c) GX-FBA predicted fluxes (blue) using the gene-expression regulation (green numbers where ↑ signifies up-regulation, and ↓ down-regulation). d) GX-FBA feasible flux region (yellow) with optimal flux state marked “GX-FBA solution.” The optimal direction is determined by the optimal FBA state and the qualitative pattern of mRNA expression (up or down).