Parental genomes have reciprocal phenotypic effects during development in the mouse because they are programmed (imprinted) with germ line-specific epigenetic modifications. These epigenetic modifications are inherited after fertilisation and they determine whether the maternal or the paternal allele of an 'imprinted' gene is expressed. Four such imprinted genes have so far been identified; the paternal genes of Igf2, and Snrpn, and the maternal genes of Igf2r and H19 are preferentially expressed during development. Igf2 and H19 are closely linked on chromosome 7 and show remarkably similar temporal and spatial patterns of expression. A mechanistic, and possibly a functional link may exist in the reciprocal imprinting of H19 and Igf2. The paternal H19 gene is apparently repressed by DNA methylation in the promoter region. This modification is not inherited from sperm but introduced after fertilisation. The nature of the primary germ line imprint therefore remains to be determined.