– GTP-Binding Protein Rheb (RHEB) is a member of the small GTPase superfamily and encodes a lipid-anchored, cell membrane protein with five repeats of the RAS-related GTP-binding region. Highest levels of RHEB can be found in the skeletal and cardiac muscle, and it is vital in the regulation of growth and cell cycle progression due to its role in the Insulin/TOR/S6K signaling pathway. RHEB stimulates the phosphorylation of S6K1 and EIF4EBP1 through activation of mTORC1 signaling, and it activates the protein kinase activity of mTORC1. RHEB has GTPase activity and shuttles between a GDP-bound form and a GTP-bound form, farnesylation of the protein is required for this activity.