Chromatin precipitation protocol was previously described (Vokes et al. 20078). Briefly, embryonic limbs were dissociated in trypsin for 5' and then crosslinked in DMEM/10%FBS/1% formaldehyde at room-temperature for 30 minutes. Cells were lysed in lysis buffer (140mM NaCl, 0.5% NP40, 0.25% Triton-X 100, 10% glycerol, 1mM EDTA, 50mM HEPES-KOH, pH7.5) at 4˚C with agitation for 10 minutes then washed with Buffer 2 (200mM NaCl, 1mM EDTA, 0.5mM EGTA, 10mM Tris-Cl pH8.0) for 10 minutes at room-temperature. Chromatin was fragmented by sonication (Branson digital sonifier with microtip) in Buffer 3 (1mM EDTA, 0.5mM EGTA, 10mM Tris-Cl pH8.0). Lysate was cleared by centrifugation and supplemented with NaCl, NaDeoxycholate, and TritonX 100 to the final concentrations of 100mM, 0.1%, and 1%, respectively. Magnetic beads (Dynal/Invitrogen, sheep anti-mouse IgG) were blocked with a blocking buffer (5mg/ml BSA in PBS) then incubated with an antibody in the blocking buffer overnight. Unbound antibody was washed away by extensive wash with the blocking buffer. Beads/antibody complex was incubated with the lysate overnight at 4°C. Beads were washed 5 times with RIPA buffer (1% NP40, 0.7% NaDeoxycholate, 0.5M LiCl, 1mM EDTA, 50mM HEPES-KOH, pH7.5). DNA was eluted with elution buffer (1% SDS, 1mM EDTA, 50mM Tris-Cl, pH8.0) at 65˚C for 15 min, and the eluate was incubated further overnight at 65˚C to reverse crosslinking. DNA was purified using PCR purification kit (Qiagen). All buffers except RIPA buffer was supplemented with protease inhibitor cocktail mix (Roche). Libraries were prepared according to Illumina protocols.