Judith Kassis, who heads the Section on Gene Expression, studies the mechanism of gene silencing by the Polycomb group genes (PcG) in Drosophila. PcG proteins are epigenetic regulators important for embryonic stem cells and for proper differentiation during development. One focus of the group is to determine how PcG proteins get recruited to the DNA. In Drosophila PcG proteins are recruited by DNA elements called Polycomb group response elements (PREs). PREs are made up of binding sites for multiple sequence-specific DNA binding proteins, but the exact nature of PREs is still not understood. The group has identified many of these PRE DNA binding proteins and are actively studying their roles in PcG recruitment. Another major project in the lab is to understand how PREs regulate the expression of the PcG-target genes engrailed and invected. These two genes are controlled by a complex array of regulatory sequences. The lab seeks to understand how gene enhancers (activating sequences), PREs, and specialized promoters elements act together to ensure the correct expression of these key developmental loci. They use genetic, genomic, biochemical, and molecular tools to manipulate regulatory DNA in vivo to understand how a regulatory element (such as an enhancer or a PRE) functions in the context of an intact locus. Their studies show the importance of genomic context and genomic neighborhood on the regulation of gene expression.