The original version of this paper contained an incorrect primer sequence. In the Methods subsection "Rampage libraries," the text for modification 3 stated that the reverse primer used for library indexing was 5'-CAAGCAGAAGACGGCATACGAGATXXXXXXXXGTGACTGGAGT-3'. The correct sequence of the oligonucleotide used is 5'-CAAGCAGAAGACGGCATACGAGATXXXXXXXXGTGACTGGAGTTCAGACGTGTGCTCTTCCGATCT-3'. This error has been corrected in the PDF and HTML versions of the paper.