Insulin-like growth factor-I (IGF-I) signalling networks in the mammary gland. Growth hormone (GH) acting on the growth hormone receptor (GHR) on stromal cells induces IGF-I release, which subsequently acts at the type I insulin-like growth factor receptor (IGF-IR) on epithelial cells to mediate survival and proliferation. Oestrogen can also induce IGF-I expression, which may then act on adjacent mammary epithelial cells. The basement membrane provides an interface between stroma and epithelial cells, and it can contribute to the signals required for mammary development via integrin receptors. Epidermal growth factor (EGF) can synergize with IGF-I, and IGF-I can transactivate the EGF receptor (EGFR). ER, oestrogen receptor.