Imprinting of the mammalian IGF2 gene. When CTC-binding factor (CTCF) binds the insulator of IGF2 (top), this enables insulator function and turns off IGF2, because the insulator is located between the promoter and the enhancer. When the insulator sequence is methylated (bottom), CTCF can no longer bind the DNA, and IGF2 can be activated by its enhancer.