Inheritance of the callipyge phenotype in sheep is an example of polar overdominance inheritance, an unusual mode of inheritance. To investigate the underlying molecular mechanism, we profiled the expression of the genes located in the Delta-like 1 homolog (Dlk1)-type III iodothyronine deiodinase (Dio3) imprinting region in mice. We found that the transcripts of the microRNA (miR) 379/miR-544 cluster were highly expressed in neonatal muscle and paralleled the expression of the Dlk1. We then determined the in vivo role of the miR-379/miR-544 cluster by establishing a mouse line in which the cluster was ablated. The maternal heterozygotes of young mutant mice displayed a hypertrophic tibialis anterior muscle, extensor digitorum longus muscle, gastrocnemius muscle, and gluteus maximus muscle and elevated expression of the DLK1 protein. Reduced expression of DLK1 was mediated by miR-329, a member of this cluster. Our results suggest that maternal expression of the imprinted miR-379/miR-544 cluster regulates paternal expression of the Dlk1 gene in mice. We therefore propose a miR-based molecular working model for polar overdominance inheritance. muscle hypertrophy | callipyge | DLK1 | miR-379/miR-544 cluster T he callipyge (CLPG) phenotype can be characterized by an increase in muscle mass due to muscular hypertrophy and a unique non-Mendelian mode of inheritance (1-3). CLPG individuals are born with normal muscle histology and develop muscle hypertrophy at ∼1 mo of age. This hypertrophy occurs primarily in the muscles of the pelvic limbs and loin and some muscles in the shoulder, which are enriched with a high proportion of type IIB fast-twitch glycolytic fibers (4). Although the molecular regulation of these hypertrophic processes is yet to be determined, Delta-like 1 homolog (DLK1) is found to be the causative protein (5-10). The unique mode of inheritance of the CLPG phenotype has attracted a great deal of attention within the scientific community. The CLPG offspring inherit the phenotype only when the mutated allele comes from the father and the wild-type allele from the mother (CLPG PAT /+ MAT ). CLPG PAT /CLPG MAT offspring exhibit a wild-type phenotype, despite carrying a CLPG mutation on the paternal chromosome. This unusual mode of inheritance led to the development of the genetic concept of polar overdominance (2, 11).The CLPG mutation has been mapped to the Dlk1-Dio3 imprinted domain, which spans ∼1 Mb and harbors at least three protein-encoding genes, including Dlk1, Peg11/Rtl1, and Dio3, and multiple noncoding RNA genes, including Gtl2, anti-Peg11/anti-Rtl1, Meg8, Mirg, C/D small nucleolar RNAs (snoRNAs), and microRNA clusters [the microRNA (miR) 379 and miR-127 clusters] (12-14). The coding genes are expressed paternally, whereas the noncoding genes are expressed maternally (15,16). Further analysis revealed a point mutation (A to G transition) within the intergenic region between the Dlk1 and Gtl2 genes of the imprinted Dlk1-Dio3 region (17, 18), which affects a muscle-specific, long-range cis-acting ...