in the early stage. Consistently, the interaction of OLIG2 with SETDB1 was most prominent in P7 mouse forebrain when massive OPCs are differentiating (Fig. 2f), which was consistent with the temporary formation of OLIG2-SETDB1 complex during OPC-iOL period when increased enrichment of H3K9me3 modification was observed in the genome (Fig. 2g). We further explored the fluctuation of SETDB1 levels in OPC lineage cells from young and old mice. As the result shown, SETDB1 decreased dramatically as mice age (Supplementary Fig. 2j). Cutting off the common H3K9me3 binding sites, the distribution of H3K9me3 enriched regions in gene body were more abundant in rat iO than that in OPCs (Fig. 2h). Genes that surrounded with iOL-specific H3K9me3 enrichment regions were very similar with genes that are downregulated upon OPCs differentiation with increased OLIG2 binding (Figs. 2i and 1b), indicating the downregulation of those genes are likely coordinated by OLIG2 and H3K9me3. Motif analysis of the H3K9me3 enrichment regions was performed, which revealed the consensus binding motif for OLIG2 in iOLs but not in OPCs (Fig. 2j). 72.2% iOL-specific H3K9me3-enrichment regions, which do not show up in OPCs, contained OLIG2 motif, while the ratio reduced to 51.1% when we considered whole H3K9me3-enrichment regions
