10e7-10e8 cells were cross-linked for 10 min in 1% formaldehyde. Lysates were sonicated to a DNA fragment length range of 200-300 bp using a Bioruptor (Diagenode). RNAPII was immunoprecipitated with 2ug of antibodies and Dynabeads Protein G (Invitrogen). Subsequently, crosslinks were reversed at 65°C over night and bound DNA fragments were purified (EZ-10 Spin Column PCR Product Purification kit, Bio Basic). Sequencing libraries were constructed using the TruSeq ChIP Sample Prep Kit (Illumina) according the manufacturer's instructions.