Cells were crosslinked with 1% formaldehyde for 15 min, and crosslinking was stopped with glycine. Cells were then lysed and whole cell lysates were sonicated to obtain DNA fragment sizes <500bp. gamma-H2AX antibody was then used to isolate DNA bound to gamma-H2AX. Libraries were prepared according to Illumina's TruSeq® ChIP Sample Preparation Guide (Part# 15023092 Rev. B) . Briefly, ChIP-enriched DNA was end-repaired using a combination of T4 DNA polymerase, E. coli DNA Pol I large fragment (Klenow polymerase) and T4 polynucleotide kinase. The blunt, phosphorylated ends were treated with Klenow fragment (32 to 52 exo minus) and dATP to yield a protruding 3- 'A' base for ligation of Illumina's adapters which have a single 'T' base overhang at the 3' end. After adapter ligation, DNA fragments with sizes of 250-300 bp were selected on 2% agarose gels and were PCR amplified with Illumina primers for 18 cycles. The libraries were captured on an Illumina flow cell for cluster generation and sequenced on HiSeq 2500 following the manufacturer's protocols.