ATAC-seq was carried out following the Omni-ATAC protocol (Corces et al Nature Methods 2017) with the following modifications. Prior to the transposition reaction, control and K27M OPC cultures were harvested and snap frozen as pellets and nuclei were isolated using Protocol 2 in the 10X Genomics Demonstrated Protocol (CG000212_SingleCellATAC_Nuclei_Isolation_MouseBrain_DemonstratedProtocol_RevB). Libraries were generated as described in the OMNI ATAC protocol (PMID: 28846090) with amplification cycles determined by a real time PCR pretest and followed by left sided clean up with 1.6X SPRISelect beads (Beckman Coulter). Libraries were analyzed for insert size distribution using the 4200 TapeStation D5000 ScreenTape assay (Agilent). Libraries were quantified  by low pass sequencing with a MiSeq nano kit (Illumina). Double end 75 cycle sequencing was performed on a NextSeq 2000 (Illumina).