cell preparation: Omni ATAC-Seq was performed on 50,000 OPCs and sorted O1+ oligodendrocytes. Nuclei were extracted from cells and treated with transposition mixture containing Nextera Tn5 Transposase for 30 minutes at 37oC with 1000 RPM mixing. Transposed fragments were then purified using Qiagen MinElute columns, PCR amplified, and libraries were purified with Agencourt AMPure XP magnetic beads with a sample to bead ratio of 1:1.2. Samples were submitted to UChicago's genomics core and sequenced on the HiSeq2500 with single-end 50bp reads with 100 million reads per sample. 