Proteomics research is uncovering a vast array of new proteins, including enzymes. It is now clear that the old assumption that one gene encodes one protein in incorrect. One gene can encode for several proteins, and many proteins have multiple functions: which function is displayed is controlled by the protein’s molecular environment. The structure and function of a protein are controlled by interactions with macromolecules such as other proteins, nucleic acids and lipids. Studying a protein in isolation (for example, obtaining the crystal or NMR structure of a purified protein) is critical to establishing a structural basis for the protein’s function(s). However, true functional characterization of the protein generally requires manipulation of multi-component systems under stringently-controlled conditions, and evaluating the roles of the physical characteristics of the protein. Proteomics has highlighted the need for a technique that can quantify the interactions between molecules under physiologically-relevant conditions.