Igf2 is an example of genomic imprinting.

In mice, the insulin-like growth factor 2 gene, Igf2, which is linked to hormone production and increased offspring growth is paternally expressed (maternally silenced) and the insulin-like growth factor 2 receptor gene Igf2r, which binds the growth protein and so slows growth, is maternally expressed (paternally silenced). The offspring is normal sized when both genes are present, or both genes are absent. When the maternally expressed gene (Igf2r) is experimentally knocked out the offspring has an unusually large size, and when the paternally expressed gene (Igf2) is knocked out, the offspring is unusually small.