Functional genomics hopes to fill the gap between sequence information and functional information. In the September issue of Nature Biotechnology, Daniel Koller and colleagues at Cytos Biotechnology AG in Zurich, Switzerland, describe the development of an expression cloning system DELphi (discovery of localized proteins) that may help to fill this gap (Nature Biotechnology 2001, 19:851-855). They attempted to overcome the limitations of current expression cloning technologies by using Sindbis virus, a single-stranded RNA virus of the Alphavirus genus. The system uses cloned cDNAs and a plasmid encoding the SIN virus-derived replicase. This is co-transfected with a helper plasmid encoding viral structural proteins to generate replication-competent bipartite virus particles, called SIN replicon particles. Koller et al. show that a single SIN particle was sufficient to initiate an infectious cycle, and infected cells expressing a receptor for a ligand of choice could be selected by flow-cytometric sorting. They validated their DELphi system by demonstrating that it could be used to clone antibody-specific antigens, and to identify receptor ligands.