The mouse Igf2 gene produces a growth factor, and homozygous mutants are dwarf mice.

Heterozygotes that receive the normal allele from their father are normal in size.

Heterozygotes that receive the normal allele from their mother, which has been imprinted, are dwarf.

The normal Igf2 gene is imprinted to function poorly during the course of egg production, but functions normally when it has passed through sperm-producing tissue in males.

This process may involve DNA methylation.