Abstract
In Drosophila, the maintenance of active and inactive patterns of gene expression during development involves the activity of two genetically complex systems. Molecular analysis of the components, apparently acting in large multiprotein complexes, has allowed a substantial advancement in our understanding of the role of chromatin higher order structures in gene regulation and nuclear organization. The Polycomb-group factors induce heterochromatin-like structures on genes that need to be stably and heritably inactivated. The role of the trithorax-group factors is to counteract these repressed chromatin domains and thus to render the genes accessible to activating factors.
