The mpkCCD cells cross-linking (formaldehyde 1.11%), nuclei were isolated and then sheared into approximately 300-bp fragments (Covaris S2 SonoLAB). Samples underwent chromatin immunoprecipitation (SimpleChIP protocol (#9003, Cell Signaling Technology) using the anti-Polr2a antibody (Catalog #: MMS-126R, Covance). After the ChIP step, crosslinking was reversed and samples were processed for library construction. The libraries were made using an Ovation Ultralow Library System (NuGen). cDNAs ranging from 200 to 400 bp were selected on 2% agarose gel and sequenced on a HiSeq2000 platform (Illumina) to generate 50-bp FASTQ sequences. The raw FASTQ sequences were mapped to the mouse Reference Genome (mm10) using the Burrows-Wheeler Aligner (BWA).