ATAC-seq was performed following the Omni-ATAC protocol {Corces, 2017}. Cells on D0, D1 and D3 were trypsinized and 50000 viable cells per condition were pelleted at 500 g for 5 min at 4⁰C. Nuclei were isolated by lysing cell pellets in 50 µl of ice-cold RSB buffer (10 mM Tris-HCl, pH 7.4, 10 mM NaCl, 3 mM MgCl2, 0.1% NP40, 0.1% Tween-20, 0.01% digitonin) and ending lysis with 1 ml RSB buffer lacking NP40 and digitonin, followed by centrifugation at 500 g for 10 min at 4⁰C. Supernatants were removed and nuclei pellets were carefully resuspended in 50 µl of transposition mixture (25 µl 2X TD buffer and 2.5 µl transposase (Illumina, 20034197), 16.5 µl PBS, 0.5 µl 1% digitonin, 0.5 µl 10% Tween-20, 5 µl H2O) to carry out the transposition reaction at 37⁰C for 30 min on a Thermomixer at 1000 rpm. Cells on D15 were washed with PBS and treated 30 min with 200U/ml DNAse I (Worthington Biochemical, LS002139) in DMEM/F12 10% FBS at 37ºC. After 4 washes with warm PBS, cells were lysed directly on the culture dish on ice using RSB containing 1% NP40, 0.1% Tween-20 and 0.01% digitonin. After a 5 min centrifugation at 500 g 4ºC, pelleted nuclei were resuspended in RSB lacking NP40 and digitonin, and then stained with Trypan Blue and counted on a Neubauer chamber. 50000 nuclei were transferred to 1ml of the same buffer to be pelleted during a 10 min centrifugation at 500g 4ºC. Tagmentation was performed as for the rest of the cells. Total tagmented DNA was purified using the Zymo DNA Clean and Concentrator-5 kit (Zymo Research, D4014) and eluted in 20 µl elution buffer. Libraries were amplified initially for 5 cycles using dual indexes for tagmented libraries from Diagenode (C01011034) with NEBNext High-Fidelity 2x PCR Master Mix (NEB, M0541). Further amplification was adjusted to the input material as assessed by qPCR using primer pair UDI 1 (Diagenode) for all samples. Library clean-up was done by performing a double-sided selection twice using AMPure XP beads (Beckman Coulter). Clean libraries were quantified with the KAPA Library Quantification Kit (Roche KK4824) and quality assessed using the Agilent TapeStation System, before sequencing at > 200 million read depth per sample with 150 bp paired-end reads on an Illumina NovaSeq.