Tracer selection for the pyruvate carboxylase flux. (A) Positive and negative sensitivities for PYC cells. The dominant sensitivity is the sum of Gluc123 and Gluc456. (B) Rules were derived to prevent overlap of the positive sensitivities with Gluc123 and Gluc456. (C) Schematic of acceptable tracers that coincide with the rules in (B). [3,4-13C]glucose was determined as the optimal tracer for the PC flux.