Imprinted genes display parent-of-origin-dependent monoallelic expression t
hat apparently regulates complex mammalian traits, including growth and beh
avior. The Peg3 gene is expressed in embryos and the adult brain from the p
aternal allele only. A mutation in the Peg3 gene resulted in growth retarda
tion, as well as a striking impairment of maternal behavior that frequently
resulted in death of the offspring. This result may be partly due to defec
tive neuronal connectivity, as well as reduced oxytocin neurons in the hypo
thalamus, because mutant mothers were deficient in milk ejection. This stud
y provides further insights on the evolution of epigenetic regulation of im
printed gene dosage in modulating mammalian growth and behavior.