ATAC-seq library preparation was performed as described (Buenrostro et al., 2013) with minor modifications. Briefly, for each experiment, 2 × 105 - 5 × 105 dissociated retinal cells were washed twice with PBS (Lonza, 17-516F) containing Proteinase Inhibitor Cocktail (Sigma-Aldrich P8340), incubated with 50 μL of lysis buffer (10 mM Tris-HCl (pH 7.4), 10 mM NaCl, 3 mM MgCl2 and 0.3% IGEPAL CA-630) on ice for 10 min and centrifuged at 500 g for 10 min at 4°C. After centrifugation, the pellet was immediately incubated with 50 μL of Tn5 transposition reaction mix (25 μL TD buffer, 2.5 μL TDE1 (Nextera DNA Library Prep Kit, Illumina, FC-121-1030) and 22.5 μL nuclease-free water) at 37°C for 30 min. After purification with QIAGEN MinElute PCR Purification Kit (QIAGEN, 28006), transposed DNA fragments were amplified with NEBNext High-Fidelity 2 × PCR Master Mix (New England Biolabs, M0541) for 5 cycles using the following PCR conditions: 72°C for 5 min; 98°C for 30 s; and thermocycling at 98°C for 10 s, 63°C for 30 s and 72°C for 1 min. 5 μL of each library was amplified in a 25 μL qPCR reaction with KAPA SYBRFAST qPCR Kit (Kapa Biosystems, KK4603) to estimate the optimum number of additional cycles to finish amplification of the remaining 45 μL library.