Regulatory associated protein of mTOR, also designated Raptor, is a binding partner for mammalian target of rapamycin kinase (mTOR), and is essential for mTOR signalling in vivo. Raptor binding to mTOR is critical for mTORcatalysed substrate phosphorylation of 4EBP1. The Raptor-mTOR complex is nutrient-sensitive and is important for a mechanism by which cells coordinate cell growth and size with changing environmental conditions. Raptor serves as a negative regulator of mTOR kinase activity under nutrient-deprived conditions and is an important component in the mTOR pathway. Raptor is highly expressed in skeletal muscle and to a lesser extent in brain, kidney, lung and placenta.