Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Phorbol esters affect blood vessel formation. In vitro differentiation of mouse embryonic stem cells leads to formation of both a primitive branched vasculature (visualized in this confocal image by PECAM antibody stain in green) and nonvascular cell types (unstained; nuclei are stained with TOPRO [blue]). This primitive vasculature is disrupted by incubation in 100 nM phorbol myristate acetate for 24 h, leading to formation of the large endothelial sheets shown. (See related article on p. 10515.)