RNA was extracted with the RNeasy mini kit (Qiagen) following manufacturer's instruction. Genomic DNA for the HA and Cbx7 ChIPs was extracted using the ChIP-IT High sensitivity kit, from Active Motif. Genomic DNA for the Ring1B ChIP was extracted according to the protocol in the Materials and Methods. IP buffer + sonication. IP buffer = (1 volume SDS buffer [100 mM NaCl, 50 mM Tris-HCl, pH 8., 5 mM EDTA, pH 8, and 2% SDS] and 0.5 volume Triton dilution buffer (100 mM NaCl, 50 mM Tris-HCl, pH 8.6, 5 mM EDTA, pH 8 and 5% Triton X-100]) with proteinase inhibitors Libraries were prepared according to Illumina instructions.