Rheb belongs to a unique family within the Ras superfamily of G-proteins. Although initially identified in rat brain, this G-protein is highly conserved from yeast to human. While only one Rheb is present in lower eukaryotes, two Rheb proteins exist in mammalian cells. A number of studies establish that one of the functions of Rheb is to activate mTOR leading to growth. In particular, the ability of Rheb to activate mTORC1 in vitro points to direct interaction of Rheb with the mTORC1 complex. Additional functions of Rheb that are independent of mTOR have also been suggested.