On the chromosome inherited from the mother, the mammalian H19 gene is active even as the upstream Igf2 gene is silenced. The reverse pattern of expression occurs on the paternal chromosome. Paternal H19 expression is silenced by promoter methylation, whereas paternal Igf2 expression relies on an enhancer located downstream of H19. In the 25 May Nature Bell and Felsenfeld, and Hark et al., find that, on the maternal chromosome, the enhancer's access to the Igf2 promoter is blocked by an intervening insulator (Nature 2000, 405:482-485; 486-489). This block is effected by the binding of the zinc-finger protein CTCF to the hypomethylated maternal insulator. The paternal insulator, however, is methylated and therefore does not bind CTCF or block interactions between the enhancer and the Igf2 promoter.