Studies of the mammalian target of rapamycin (mTOR) and its homologs demonstrate a role in integrating signals from growth factors, nutrients, stress, and cellular energy levels to control cell growth, translation initiation, ribosome biogenesis, and transcription factor localization. mTOR is the direct target of the cell cycle arresting activity of rapamycin. mTOR interacts with Raptor or Rictor to form the mTORC1 and mTORC2 complexes respectively. The mTORC1 complex also includes mLst8/GbetaL and functions to phosphorylate S6K and 4EBP1. The mTORC2 complex also includes mLst8/GbetaL, mSIN1 and protor-1 and functions to phosphorylate Akt.