Metabolism is the ensemble of chemical transformations carried out in living tissue; operationally it is embodied in the matter and energy fluxes through organisms. Metabolomics is defined as the measurement of the amounts (concentrations) and locations of the all the metabolites in a cell, the metabolites being the small molecules (M r . ≲ 1000, e.g. glucose, cAMP,1 GMP,2 glutamate etc.) transformed in the process of metabolism, i.e. mostly the substrates and products of enzymes.3 The quantification of the amounts of expressed enzymes is, as we have seen, proteomics; metabolomics is essentially an extension of proteomics to the activities of the expressed enzymes, and it is of major interest to examine correlations between expression data and metabolite data.4