Imprinted expression at the H19-Igf2 locus depends on a differentially methylated domain (DMD) that acts both as a maternal-specific, methylation-sensitive insulator and as a paternal-specific site of hypermethylation. Four repeats in the DMD bind CCCTC-binding factor (CTCF) on the maternal allele and have been proposed to attract methylation on the paternal allele. We introduced point mutations into the DMD to deplete the repeats of CpGs while retaining CTCF-binding and enhancer-blocking activity. Maternal inheritance of the mutations left H19 expression and Igf2 imprinting intact, consistent with the idea that the DMD acts as an insulator. Conversely, paternal inheritance of these mutations disrupted maintenance of DMD methylation, resulting in biallelic H19 expression. Furthermore, an insulator was established on the paternally inherited mutated allele in vivo, reducing Igf2 expression and resulting in a 40% reduction in size of newborn offspring. Thus, the nine CpG mutations in the DMD showed that the two parental-specific roles of the H19 DMD, methylation maintenance and insulator assembly, are antagonistic.