library_construction_protocol: Cells were fixed for 10 minutes in 1% formaldehyde, quenched, and frozen. Chromatin was sonicated with a Bioruptor Plus (Diagenode) at high power for 30 seconds on and 30 seconds off. Sixty cycles was chosen to fragment DNA with a peak of fragments ~200 - 600 bp. 100 µg chromatin as measured from the decrosslinked sample was aliquoted to each ChIP reaction. Eluted DNA was decrosslinked and purified using Monarch PCR and DNA Cleanup Kit (New England Biolab T1030S). DNA was quantified using Qubit HS dsDNA and High Sensitivity D5000 DNA ScreenTape assays (Agilent). Sequencing libraries were generated using NEBNext Ultra II (E7645S) with NEBNext multiplex oligos for Illumina (E7335S). The number of PCR cycles was selected according to manufacturer's recommendations for the mass of DNA in each library. Input libraries were amplified with 3 cycles of PCR, while H4K20me1 libraries using the Active Motif or Thermo antibodies required 6 cycles of PCR. QC of sequencing libraries was performed using Qubit High Sensitivity dsDNA assays for DNA concentration and High Sensitivity D1000 and D5000 DNA ScreenTape assays (Agilent)  for fragment length distributions. Libraries generated using the Active Motif or Thermo H4K20me1 antibodies were size-selected them with SPRISelect beads (Beckman Coulter B23317) prior to sequencing, using a right-side size selection with 0.4x bead ratio to remove large fragments.