Plays an essential role in the protection and maintenance of transcriptional and developmental programs. Protects many bivalent promoters and poised enhancers from hypermethylation, showing a marked preference for these regulatory elements over other types of promoters or enhancers. Mechanistically, cooperates with tet1 and binds to DNA in a common complex to inhibit the binding of dnmt3a/3b and therefore de novo methylation.