Clinical and experimental data support a role for insulin-like growth factor I (IGF-I) in breast cancers. Our laboratory previously showed that IGF-I is a stimulus for mammary gland branching morphogenesis as well as breast tumor growth and metastasis. Recent epidemiological studies indicate that elevated serum IGF-I levels in premenopausal women are linked with increased mammographic density, ductal carcinoma in situ and breast cancer risk. The IGF-I receptor is reportedly overexpressed in 30–40% of early-stage breast cancers. It is unclear why activation of IGF-I receptor signaling is associated with the development of cancer. We are using complementary DNA microarrays to profile gene expression in normal breast epithelial cells and to investigate the mechanism behind cancer risk related to IGF-I. We profiled the gene expression of normal breast epithelial cells in the absence or presence of IGF-I at 0, 0.5, 1, 2, 4, 6, 8 and 24 h on 2K cDNA microarray chips and observed inhibition of pro-apoptotic genes, including fas ligand. We also found increased expression of invasion and neovascularization genes. For example, the potent vascular mitogen VEGF was induced by IGF-I at the 2-h time point. To confirm differential gene expression, messenger RNA was validated by a real-time quantitative polymerase chain reaction, and the protein products were confirmed by enzyme-linked immunosorbent assay. Cellular trafficking was studied to illustrate that the cells were responding to IGF-I through a PI3K/AKT pathway. We demonstrate that the activated forms of AKT, GSK, CREB and HIF-1a translocated into the nucleus in response to IGF-I treatment. The differential gene profiling of normal breast epithelial cells to growth factors has not been reported to date, and this investigation provides insight into the expression of gene products that may have fundamental importance to the development and progression of breast cancer.