Biology Department, Syracuse University, Syracuse, NY, USA rhis review compares the signaling pathways leading to cellular responses (primarily proliferation and differentiation) of cells to the insulin-like growth factors (IGF5). Although some systems (such as myoblasts and adipocytes) clearly employ the Ras-Raf-Mitogen Activated Protein (MAP) kinase pathway in signaling for cell proliferation, others (such as MCF-7 mammary tumors and brain capillary cells) proliferate in response to signals mediated by phosphatidylinositol-3 kinase and p70 56 ki-nase. Similarly, most of the systems surveyed use a phosphati-dylinositol-3 kinase pathway in differentiating in response to IGFs, but others (such as SH-SY5Y neuroblastoma cells) differentiate in response to the MAP kinase pathway. Thus, it seems that there are no simple generalizations that can be used to forecast the signaling pathway that will be involved in any re-sponse to the IGFs.