Euchromatin and heterochromatin. Unmethylated CpG islands (blank circles), permissive histone modifications (green stars) and loose chromatin structure promote gene transcription in the euchromatin state. Conversely, DNA methylation (red circles), repressive histone modifications (red stars) and condensed structure prevent transcription in the heterochromatin state. Although governed by distinct enzymes, cooperativity and interaction between the different epigenetic modifications provide a self-reinforcing mechanism for epigenetic regulation. TET=Ten-eleven translocation dioxygenase; DNMT=DNA methyltransferase.