Cover photograph (Copyright © 1999, American Society for Microbiology. All rights reserved.): Two classes of genes in Drosophila, the trithorax group and the Polycomb group, are required in concert to maintain gene expression by regulating chromatin structure. Here an eye color assay in transgenic flies is used to examine the effect of trithorax mutations on the activity of its regulatory element in the bithorax complex. (See related article on page 5189.)