To isolate chromatin, formaldehyde-cross-linked nuclei were sonicated using a Branson 450 Sonifier (Branson, Danbury, CT). Following ChIP performed with Dynal beads (Invitrogen, Carlsbad, CA), ChIP DNA was subjected to end repair, A-tailing, adaptor ligation, and gel purification. Following PCR amplification, the final library underwent gel purification.