Cells were isolated in PBS supplemented with 10% Fetal Calf Serum. ATAC experiments were performed according to Buenrostros et al., (2013) protocol and using a homemade transposome from Picelli et al., (2014). 7-8 embryos at 10-12s were dissected in cold PBS for each genotype and cells were mechanically dissociated. Cells were lysed before transposition using 1µL of transposome and purified using a Qiagen MinElute Kit with 10µL of Elution Buffer. Transposed DNA was amplified by PCR as previously described and quantified by qPCR using 5µL of PCR products. The number of additionnal cycles is determined by plotting linear Rn versus cycle and correspond to the one-third of the maximum fluorescence intensity. The remaining 45µL of PCR products are runned with the additional number of cycles. The final product is purified with Qiagen PCR Cleanup Kit and eluted in 20µL Elution Buffer. Sequencing was done on multiplexed samples using 42 bp Paired-end reads on the Illumina NextSeq system according to the manufacturer’s specifications at the genomics platform of IBENS.