Tracer selection for the oxidative pentose phosphate pathway flux. (A) Positive and negative sensitivities for PYC cells. The dominant sensitivity is Gluc123 and the rules in (B) were derived to maximize this negative sensitivity. The rules are listed in sequential order of importance. (C) Schematic of acceptable tracers that coincide with the rules in (B). [2,3,4,5,6-13C]glucose was identified as the optimal tracer for the oxPPP flux.