RPTOR is a 150 kDa component of the cytosolic mammalian target of Rapamycin complex 1 (mTORC1) which also contains mTOR and GBL. mTORC1 plays a dominant role in cell cycle regulation in response to metabolic conditions. The interaction of RPTOR with the kinase mTOR is stabilized under conditions of nutrient deprivation and energy stress, leading to inhibition of mTOR and cell cycle arrest. RPTOR contains multiple Ser and Thr residues whose phosphorylation regulates the activation status of mTOR. RPTOR is critical for the response of skeletal muscle and adipose tissue to insulin.