Cover photograph (Copyright © 2003, American Society for Microbiology. All Rights Reserved.): Binding of substrates to AcrB multidrug efflux transporter of Escherichia coli. AcrB pumps out a very wide range of compounds, including most antibiotics, disinfectants, detergents, and cationic dyes. X-ray crystallography showed that its substrates are bound to the wall of the large central cavity between the transmembrane domains of the three protomers of AcrB. Positions of binding of three ligands are shown in stick models: ciprofloxacin (yellow), ethidium (green), and dequalinium (orange). This is a composite figure, however, and the three different drug molecules may not bind to a single cavity simultaneously (See related article on page 5657.)