The invention relates to the finding that animals deficient in the Peg3 gene suffer from a number of phenotypic traits, including obesity, aberrant thermoregulation and behavioral defects, including impaired maternal behavior. The invention provides a transgenic non-human animal which comprises an inactive copy for the Peg3 gene, and the use of such animals as model systems in assays for novel therapies in the treatment of conditions such as those mentioned above.