The biological actions of the insulin-like growth factors IGF-I and IGF-II are mediated by their activation of the IGF-IR, a transmembrane tyrosine kinase linked to the ras-raf-MAPK cascade. Functional IGF-IRs are required for the cell to progress through the cell cycle. Most importantly, cells lacking this receptor cannot be transformed by any of a number of dominant oncogenes, a finding that proves that the presence of the IGF-IR is important for the development of a malignant phenotype. Consistent with this role, the IGF-IR displays a potent antiapoptotic effect, both in vitro and in vivo. Because of its key role in the transformation process, the IGF-IR is actively studied as a potential therapeutic target in different types of neoplastic growth.