I recently identified all the protein components of the Drosophila PRC1 complex, establishing an unexpected connection between Polycomb group (PcG) proteins (which are involved in maintaining the repressed state of the homeotic genes during development) and components of the general transcription machinery. Other non-PcGproteins were also identified, including a sequence-specific DNA binding protein. These proteins are associated with the PcG proteins in PRC1 from 0-12h Drosphila embryos, which encompass a large portion of embryonic development. I propose to now study the composition of the complex at more defined stages of embryonic development, and to equate this composition to PcG activity using pre-established assays. A crucial time for the correct development of the Drosophila embryo is at approximately 5 hours, when the PcG take over the role of maintaining homeotic gene repression from factors that previously set up these patterns of expression. How the PcG recognize the genes it needs to maintain in a repressed state at this time is unknown, although is believed to occur through a network of recruitment through protein-protein interactions. Examination of the protein composition of the PRC1PcG complex purified from embryos staged at, and around, this important time in developmental should allow us to better understand how this family of proteins is able to associate with the genes they repress.