Accordingly, high quality ChIP-seq reads that had failed to align to the mouse reference genome (i.e. unmapped reads) were aligned to a custom reference genomic library containing these exogenous genomic sequences ( Fig. 2b). Sequence Alignment/Map tools (SAMtools 0.1.18; [16]) were used to generate FASTQ files for the unaligned reads, which were then aligned to the aforementioned reference library using the ELAND2 standalone aligner. Peak finding was executed on the resulting BAM files using MACS 1.4.2 to generate WIG files with the genome-size setting modified to represent the size of the reference used for alignment. The resulting peak data was then further examined and graphically represented MATLAB (R2013b).