At the International Conference and Exhibition on Metabolomics & Systems Biology the IROA protocol was discussed. Isotopically signed cells, i.e. cells in which all of the biomolecules (DNA, RNA, and metabolites) are labeled with IROA media, carry distinctive isotopic signatures. Since these signatures may be distinguished from one another, artifacts may be removed, and multiple samples prepared and analyzed in a single analytical process, thereby reducing all sample-to-sample variances, including ion suppression, and increasing data quality. The signature for each molecule also carries information defining its molecular structure making its identification less prone to error. The significantly enhanced quantitation of having a biochemically complex internal standard present in every experimental sample is an additional benefit of the IROA protocol. We will present the development of toxicological mechanism of action (MOA) profiles using an IROA protocol. In yeast, S. cervisiae S288C, the MOA to flucytosine is clearly and easily seen. Additional metabolic pools that are secondarily effected by the primary mechanism of toxicity are also easily determined. Thus the full biological response to the toxin may be easily defined.