Genetic and embryological studies in the mouse demonstrated functional differences between parental chromosomes during development^1–3. This is due to imprinted genes whose expression is dependent on their parental origin^4,5. In a recent systematic screen for imprinted genes, we detected Peg3 (paternally expressed gene 3)⁶. Peg3 is not expressed in parthenogenones. In interspecific hybrids, only the paternal copy of the gene is expressed in the embryos, individual tissues examined in d9.5–13.5 embryos, neonates and adults. Peg3 mRNA is a 9 kb transcript encoding an unusual zinc finger protein with eleven widely spaced C2H2 type motifs and two groups of amino acid repeats. Peg3 is expressed in early somites, branchial arches and other mesodermal tissues, as well as in the hypothalamus. Peg3 maps to the proximal region of chromosome 7. Consistent with our findings, maternal duplication of the proximal chromosome 7 causes neonatal lethality^7–9. This region is syntenic with human chromosome 19q13.1–13.3 (refs 10,11), where the genes for myotonic dystrophy and a putative tumour suppressor gene are located^12,13.