Mammary gland branching morphogenesis is diminished in mice with a deficiency of insulin-like growth factor-I (IGF-I), but not in mice with a liver-specific deletion of IGF-I.

The development of the mouse mammary gland occurs postnatally. Hormonal activation of local growth factor pathways stimulates rapid elongation and branching of the rudimentary gland through the fatty stroma. Earlier studies showed that GH is required for mammary gland ductal morphogenesis and that IGF-I mediates this action of GH.