mTOR (Mammalian Target of Rapamycin, FRAP, RAPT or RAFT) is a large 289kD Ser/Thr protein kinase that regulates cell cycle progression, cell growth, protein synthesis, ribosome biogenesis, and autophagy. mTOR is an evolutionarily conserved member of the Phosphoinositol Kinase-related Kinase (PIKK) family whose activity is regulated by phosphorylation on Ser2448 by Akt in response to insulin or muscle activity. mTOR is the central component of two multimeric kinase complexes consisting of mTOR and numerous other mTOR binding proteins. These two multimeric protein complexes are designated mTORC1 and mTORC2. mTORC1 consists of at least mTOR, Raptor, and G betaL. mTORC1 is known to play a central role in insulin signaling, which is crucial in maintaining metabolic homeostasis. The other mTOR complex, mTORC2, is made up of mTOR, Rictor, G betaL, Sin1, Protor 1 and 2. TORC2 affects cell proliferation and survival primarily by phosphorylating the hydrophobic motif of Akt on Ser473. TORC2 complex is also known to effect cytoskeletal organization and migration by exerting its effects through Rac, Rho, and PKC. Defects in both mTOR complexes are associated with a variety of diseases, including cancer and diabetes.