Cover photograph (Copyright © 2002, American Society for Microbiology. All Rights Reserved.): Sagittal section of an embryonic day 13.5 heterozygous Pop1-LacZ mouse embryo stained for -galactosidase activity and counterstained with eosin. At this stage of development Pop1-LacZ is predominantly expressed in the myocardium. Prolonged incubation of the enzyme reaction resulted in the visualization of additional expression domains, including dorsal root ganglia and smooth muscle cells in various organs. (See related article on p. 1504.)