Genomic imprinting confers allele-specific expression in less than 1% of genes, in a parent-of-origin specific fashion. In humans and mice the Peg1/Mest gene (Mest) is maternally repressed, and paternally expressed. Mest is expressed in embryogenic mesoderm-derived tissues and in adult brain, and paternal mutations in Mest lead to growth retardation and defective maternal behaviour. Despite our current understanding of mechanisms associated with the establishment of imprinting of Mest and other imprinted genes, it is unclear to what extent Mest imprinting needs to be maintained in adult tissues. Aberrations of imprinting are known to occur in certain rare syndromes, and involve either inherited mutations, or constitutive epigenetic alterations occurring soon after fertilization. Imprinting abnormalities may also occur in the aging somatic tissues of adult individuals. Here we report an occurrence of post-embryonic somatic variability of Mest allelic expression in a colony of mice where heterozygotes at the imprinted Mest locus for a mutation inherited from the father spontaneously expressed the normally silenced allele from the mother. In addition, a newly acquired ability to overcome the deficit in maternal reproductive behaviour had occurred in the mutant mice, but this appeared not to be directly linked to the Mest mutation. Our results suggest that at least one allele of Mest expression is required in the somatic tissues of adult individuals and that under certain conditions (such as in the presence of a Mest insertional mutation or in an altered genetic background), somatically acquired alterations of allelic expression at the Mest locus may occur.