Cells were cross-linked with formaldehyde at 1% final concentration. Nuclei were extracted in low salt cnditions and burst in high salt before shearing the DNA to the reduced size of 300-500bp. ChIP was performed with antibodies against RNA Pol II (N20, santa cruz) or GFP/Flag-tagged CSB (GFP, abcam). Chip-Seq: DNA samples were end repaired, poly-A tailed and Illumina single end adapters were ligated following the standard Illumina protocol with minor adjustments. Agencourt AMPure XP beads at 0.8x ratio were used to size select out adapter dimers after adapter ligation. The Illumina kit Phusion enzyme was replaced by Kapa HiFi HotStart ready mix. Post PCR, AMPure XP beads were used at a 1:1 ratio to maintain size integrity and to allow use of the Invitrogen SizeSelect E-gel system. Samples were finally purified with QAIquick gel extraction kit and quality controlled on the DNA 1000 BioAnalyser 2100 chip before clustering and subsequent 36bp single end sequencing on the Illumina Genome Analyzer IIx.