ATAC–seq was performed as described in (Buenrostro et al., 2013). Micro-dissected tissues (a pool of 2 GT or 2 to 3 CR) were isolated in PBS supplemented with 10% Fetal Calf Serum and dissociated to single cell by collagenase treatment. After isolation, 50,000 cells were lysed in 50 μl of lysis buffer (10 mM Tris-HCl, pH 7.4, 10 mM NaCl, 3 mM MgCl2 and 0.1% IGEPAL CA-630), nuclei were carefully resuspended in 50μl transposition reaction mix (25μl TD buffer, 2.5μl Tn5 transposase and 22.5μl nuclease-free water) and incubated at 37 °C for 30min. DNA was isolated with a MinElute DNA Purification Kit (Qiagen). Library amplification was performed by PCR (10 to 12 cycles) using NEBNext High-Fidelity 2x PCR Master Mix (NEB, M0541S). Library quality was checked on a fragment analyzer, and paired-end sequencing was performed on an Illumina NextSeq 500 instrument (read length 2 × 37 base pairs).