Chromatin immunoprecipitation was performed with 1x10^7 cells per experiment. The cells were collected and cross-linked for 15min in 1% paraformaldehyde, washed and lysed. Chromatin was sheared using a Bioruptor to create fragments of approximately 150 base pairs, incubated with about 2–5 μg antibody bound to 75 μl Dynabeads M-280 (Invitrogen) and rotated overnight at 4 °C, then washed and eluted. The eluted chromatin was reverse-cross-linked and column purified. ChIP samples were prepared for sequencing using Illumina TruSeq DNA Sample Preparation Kit according to the standard preparation protocol.