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Judith Kassis, who heads the Section on Gene Expression, studies the mechanism of gene silencing by the Polycomb group genes (PcG) in Drosophila and the nature of DNA elements responsible for silencing (called Polycomb response elements or PREs). PREs are several hundred nucleotides in length and consist of binding sites for numerous proteins. The laboratory uses two approaches to understand PRE function. First, the group identifies DNA sequences required for PRE function and has made considerable progress towards identifying the complex array of DNA binding proteins required for the activity of one PRE. Second, in an effort to understand the role of PREs in the context of other regulatory DNA, the Section studies PRE activity at the PcG target gene engrailed. The DNA that regulates engrailed expression stretches over 70kb. The Section's recent results indicate that the chromatin structure set up by the PcG proteins may facilitate the activity of some engrailed enhancers.