The imprinted H19 gene, which encodes an untranslated RNA, lies at the end of a cluster of Imprinted genes in the mouse. Imprinting of the insulin-2 and insulin-like growth factor 2 genes, which lie about 100 kilobases upstream of H19, can be disrupted by maternal inheritance of a targeted deletion of the H19 gene and its flanking sequence. Animals inheriting the H19 mutation from their mothers are 27% heavier than those inheriting It from their fathers. Paternal inheritance of the disruption has no effect, which presumably reflects the normally silent state of the paternal gene. The somatic overgrowth of heterozygotes for the maternal deletion is attributed to a gain of function of insulin-like growth factor 2, rather than a loss of function of H19.