The mouse l23mrp gene located downstream of the imprinted ins2/igf2/h19 region is biallelically expressed and not influenced by the h19 enhancer

The Ins2 (insulin II), Igf2 (insulin-like growth factor II) and H19 genes are closely linked on mouse chromosome 7 and all are subject to tissuespecific parental imprinting. It was previously shown that the imprinting of these genes involves their parental-origin-specific interactions with the endoderm-specific H19 enhancer. To know the extent and boundaries of this imprinted domain, we have cloned and characterized the mouse homolog of the human L23MRP gene, which had been mapped approximately 40 kb downstream of the human H19 gene. Our findings show that the mouse L23mrp gene is expressed biallelically in various developmental stages and tissues. On the basis of the analysis of H19 enhancer deletion mice, we also conclude that the enhancer does not influence L23mrp expression. Thus it is likely that this gene demarcates the 3' end of the imprinted domain. The lack of interactions between the enhancer and the gene may be due to either the promoter specificity of the enhancer or to the presence of a putative chromatin insulator.