Dissected tissue from embryos was pooled and turned into a single-cell suspension by digestion with Trypsin-EDTA 0.05% (Gibco) for 10 min at 37°C shaking at 900 RPM. Drosophila melanogaster S2 cells were spiked in in a 1:3 ratio. The cells were mixed with 10%FCS/PBS and homogenized using a 40 µm cell strainer (Falcon). After centrifugation, cells were fixed in 1% PFA/10%FCS/PBS for 10' at room temperature. Cells were then lysed 3C-Lysis buffer (50mM TRIS pH 7.5; 150mM NaCl; 5mM EDTA; 0.5% NP-40; 1.15% Triton X-100; Protease Inhibitors (Roche)) and nuclei pelleted by centrifugation. For sonication, nuclei were resuspended in buffer 3 (Lee et al. 2006). Chromatin was sheared using a Bioruptor until reaching a fragment size of 200-500bp. Lysates were clarified from sonicated nuclei and protein-DNA complexes were isolated with the respective antibody. 10-15μg of chromatin was used for each replicate ChIP. Antibodies: H3K27ac (Diagenode, C15410174) as in Ibrahim et al. 2013. Sequencing libraries were prepared using Nextera adaptors.