The ultimate goal of the various genome sequencing projects is to gain sufficient understanding of the physiology of the organisms under study to enable exploitation of the genetic information to advance human health, agricultural production and industrial fermentation. A list of genes alone, however, is unlikely to provide sufficient information to manipulate the metabolism or pathophysiology of an organism in a predictable way. In this issue, Schuster et al. describe a method, called metabolic flux analysis, that is designed to convert a list of putative enzymes into a set of metabolic pathways. Their approach involves a rigorous definition of the constitutive activities of a given pathway as the sum of “elementary flux modes”, each of which represents a minimal sequence of metabolic steps that can operate independently of each other. Applications should include more efficient identification of important drug targets and assigning and/or corroborating specific activities to orphan gene sequences (see p. 326).