Cell growth is a fundamental biological process whereby cells accumulate mass and increase in size. The mammalian Target of Rapamycin (mTOR) pathway regulates growth by coordinating energy and nutrient signals with growth factor-derived signals. mTOR is a large protein kinase with two different complexes. One complex contains mTOR, GβL, and raptor, which is a target of rapamycin. The other complex, insensitive to rapamycin, includes mTOR, GβL, and rictor. GβL associates with the kinase domain of mTOR and stimulates mTOR kinase activity. A reduction in GβL expression has been shown to decrease in vivo phosphorylation of S6K1.