Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Nuclear localization of the LasR bacterial transcriptional regulator. Pseudomonas aeruginosa uses quorum sensing, mediated by homoserine lactones, to control gene expression in response to certain environmental cues. The effect of homoserine lactones on eukaryotic host cells is the subject of new research. The ability of these chemicals to enter and function in host cells suggests a new means of cross-kingdom signaling, which may provide new insights into pathogenesis and infection. Shown is an immunofluorescence image of a monkey COS-1 cell expressing a LasR chimeric protein. (See related article on page 2281.)