 The ADD domain serves as an inhibitor of the methyltransferase domain until "DNMT3A" binds to the unmodified lysine 4 of histone 3 (H3K4me0) for its "de novo" methylating activity.This protein thus seems to have an inbuilt control mechanism targeting histones only for methylation.Finally, the methyltransferase domain is highly conserved, even among prokaryotes!