SETDB1 is a tri-methyltransferase of H3K9. The binding between OLIG2 and SETDB1 was validated by immunoprecipitation (Supplementary Fig. 2d, e). As Ehmt2 is a specific H3K9me2 (but not H3K9me3) transferase according to previous studies25, we further examined if interactions between OLIG2 and EHMT1, SUV39h1 or PRDM2 exist, and we hardly detected any (Supplementary Fig. 2f). We further checked the integrity of OLIG2-BRG1 transcriptional activation complex in Setdb1 knockdown rat iOLs. And we observed that the interaction between OLIG2 and BRG1 was barely affected in the absence of SETDB1 (Supplementary Fig. 2g). In mouse brain, protein levels of SETDB1 and H3K9me3 were higher in differentiated oligodendrocytes (GSTπ+ or CC1+) than that in OPCs (OPC: PDGFRα+)(Supplementary Fig. 2h, i). Notably, SETDB1 expression decreased rapidly in fully matured oligodendrocytes (iOL: CC1+/MBP-; mOL: CC1+/MBP+) (Fig. 2e), which suggests that SETDB1-mediated H3K9me3 modification mainly functions 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
