library_construction_protocol: Nuclei from hippocampal cells were extracted in a dounce homogenizer as explained for the FANS method (see above). For H3K4me3 (07-473, Millipore), nuclei were fixed in 1% formaldehyde for 10 min at room temperature, and fixation was blocked with 0.1 M glycine for 5 min. The nuclei pellet was resuspended in SDS lysis buffer (containing, in mM: SDS 1%, EDTA 10, Tris 50; pH 8) and sonicated in a Bioruptor Pico (Diagenode) for 12 cycles of 15 s with 30 s off. Samples were centrifuged for 6 min at 13000 rpm at room temperature and the supernatants were kept in LoBind Eppendorf tubes. 10% of the samples were saved for quality control analysis (DNA concentration measurement and visualization of smir DNA in an electrophoresis gel) and the rest was diluted in ChIP Dilution buffer (containing, in mM: SDS 0.01%, Triton X-100 1.1%, EDTA 1.2, Tris-HCl 16.7, NaCl 167; pH 8). 5% of the sample was saved as Input and the rest incubated overnight at 4°C with the Dynabeads (Invitrogen) bound to the antibody. Samples were rinsed with several washes, 5 minutes each, as follows: 2x in RIPA-150 buffer (containing, in mM: Tris-HCl 50, NaCl 150, EDTA 1, SDS 0.1%, Triton X-100 1%, and sodium deoxycholate 0.1%; pH 8), 2x in RIPA-500 buffer (containing, in mM: Tris-HCl 50, NaCl 500, EDTA 1, SDS 0.1%,  Triton X-100 1%, sodium deoxycholate 0.1%; pH 8), 2x in RIPA LiCl buffer (containing, in mM: Tris-HCl 50, EDTA 1, NP-40 1%, sodium deosycholate 0.7%, LiCl2 500), and 2x in TE buffer (containing, in mM: Tris-HCl 10, pH 8.0, EDTA 1; pH 8.0). Samples were diluted in Elution buffer (containing, in mM: Tris-HCl 10, 5 mM EDTA, NaCl 300, SDS 0.5%; pH 8) and incubated with 1 μL of RNase A 10 mg/mL (Fermentas) overnight at 65°C in agitation to reverse crosslink. Samples were then placed on a magnetic rack, allowing beads to clump. Supernatants were transferred to new tubes and incubated with 2 μL of Proteinase K 20 mg/mL (Thermo Scientific) for 4 h at 55°C, in agitation. Quality controls and Inputs were also incubated with RNase A and Proteinase K. Finally, DNA was precipitated using phenol-chloroform-isoamil alcohol. ChIP-seq