mTOR (mammalian target of rapamycin) is a 289 kD member of the PI3 kinase family, containing HEAT, FRB, FATC domains. mTOR is expressed in the nucleus and cytoplasm and is a target for cell-cycle arrest and the immunosuppressive effects of FKBP12-rapamycin complex. mTOR controls ribosome biogenesis and cell growth and acts as an ATP sensor. mTOR is repressed by nutrient deprivation and mitochondrial uncoupling. Association of raptor negatively regulates mTOR kinase activity, also inhibited by tuberin-hamartin tumor suppressor complex. mTOR is the target of PI3K-regulated protein kinase, AKT/PKB (phosphorylated at Thr 2446 and Ser2448). mTOR has also been shown to bind RAPTOR which negatively regulates mTOR kinase activity. The binding of RAPTOR to mTOR is necessary for the mTOR-catalyzed phosphorylation of 4EBP1 in vitro, and it strongly enhances the MTOR kinase activity toward p70-alpha.