Genomic imprinting describes a process by which a subpopulation of genes are epigenetically marked such that those genes are differentially expressed in the conceptus depending on whether they were inherited from the maternal or paternal gamete. Elucidating the role of imprinting in mammalian development and evaluating the involvement of imprinting in human disease, particularly in the context of imprinted genes as potential targets for environmental agents, will require a thorough understanding of the mechanism(s) by which gene-expression imprints are set, maintained, and erased. A number of molecular models have been proposed to explain how gene-expression and methylation imprints are created and realized, and there is an emerging concensus that chromatin conformation may be an integral part of the imprinting process. This pilot study will determine whether mouse mutations can be recovered that affect the mechanism(s) of imprinting. Specifically, a new deletion model of mouse Prader-Willi Syndrome (muPWS) will be exploited as a visible reporter to screen for N-ethyl-N-nitrosourea (ENU)-induced dominant and recessive heritable mutations in the process(es) that govern the imprinting switch in the female germ line. The single-base-pair changes associated with the majority of ENU- induced mutations should be particularly valuable for generating dominant-negative mutations (perhaps of a protein complex necessary for correct imprinting, no matter what the mechanism) and hypomorphic and/or """"""""leaky"""""""" mutations in the recessive screen that may disrupt imprinting without compromising basic cellular processes to the point of causing a lethality. If such imprinting mutations can be ascertained, they would provide important new reagents for studying the complicated intermolecular interactions that presumably govern the overall process of imprint creation, maintenance, and erasure. The availability of stably inherited, one-gene variation in the imprinting process (on a widely used inbred mouse background) should be useful for future molecular dissection of imprinting mechanism(s), and would likely inform other groups actively investigating the effect of environmental agents on imprinting as to which pathways and specific protein products or protein complexes may be targets for the corresponding agent.