Maxillary arches were dissected from 47 of E11.5 CD-1 wild type embryos in phosphate-buffered saline (PBS), and snap-frozen on dry ice The tissue was then sent to Active Motif, Inc. (Carlsbad, CA) for HistonePathTM service, which included chromatin preparation, ChIP-seq for H3K27ac, and bioinformatics analysis of the sequencing result to identify H3K27ac-enriched genomic regions (peaks). 20 μg of the maxillary arch chromatin was used for ChIP with a rabbit polyclonal anti-H3K27Ac antibody (Active Motif, cat# AM 39133). Libraries for Illumina sequencing (Hi-Seq) were prepared from ChIP DNA and input chromatin as described (Labhart et al., 2005)