HoxB8 cells were fixed with either 1% or 0.4% formaldehyde for 10 mins. Chromatin was isolated, sonicated for 7 mins (30 sec on, 30 sec off), and specific antibodies were used to pull down the transcription factors of interest after a pre-clearing step. Chromatin was washed, de-crosslinked, amplified, size selected by gel purification and sequenced. As described in Wilson et al., Cell Stem Cell. 2010 Oct 8;7(4):532-44 (PMID 20887958). Library construction: According to Illumina TruSeq DNA Sample Prep Kit.