Mammalian target of rapamycin(mTOR)exists downstream in the PI3K/AKT signal transmission pathway that is constantly activated in many cancers. It regulates translation of mRNA that codes for proteins involved in survival signals, such as for the cell cycle and apoptosis, by phosphorylation of p70S6K and 4E-BP1 that are further downstream. Thus, mTOR promotes cell proliferation and translation. It is also related to the growth and proliferation of cancer vascular endothelial cells. Inhibition of mTOR has been shown to lead to the inhibition of cancer cell proliferation and cancer angiogenesis, which are activated by mTOR, and to have an antitumor effect. In recent years, advances have been made in the development of drugs that target mTOR; that is, mTOR inhibitors. These drugs are being incorporated into the first and second-line therapies as standard treatment for renal cell cancer. The usefulness of mTOR has also been shown in pancreatic neuroendocrine tumor. The next task is to discover a clinically effective biomarker that predicts effect and toxicity of mTOR.