Normal mammalian development requires a diploid combination of both haploid parental genomes¹. Uniparental disomy for certain segments of specific chromosomes results in aberrant development or prenatal lethality^2,3, indicating that the parental genomes have undergone modifications during gametogenesis. These modifications result in parent-of-origin specific expression for some genes, a phenomenon called genomic imprinting. Recent work with DNA methyltransferase deficient mice showed that differential methylation is the probable basis of the imprinted character of several genes⁴. Screening for endogenous imprinted loci using restriction landmark genomic scanning with methylation sensitive enzymes (RLGS-M) identified eight imprinted RLGS (Irlgs) candidate loci^5,6. Molecular analysis of the genomic region of one of the loci (Irigs2) resulted in the discovery of the paternally imprinted U2afbp-rs gene within a previously identified imprinted region on mouse chromosome 11 (refs 5, 7). This paper describes the characterisation of a novel imprinted RLGS-M locus, Irlgs3, on mouse chromosome 9 (ref. 6). Within this locus we identified the Grf1 (also called Cdc25^Mm) gene, which is homologous to the RAS-specific guanine nucleotide exchange factor gene, CDC25, in Saccharomyces cerevisiae. Grf1 is located about 30 kb downstream of the methylation imprinted site, identified by RLGS-M, and shows paternal allele specific expression in mouse brain, stomach and heart. Our results indicate that imprinting may have a role in regulating mitogenic signal transduction pathways during growth and development.