mTORC1 kinase complex is a critical component in regulating cell growth (1,2). Its activity is modulated by energy levels, growth factors and amino acids (3,4). Recent studies found that RagA, RagB, RagC and RagD, the four related GTPases, interact with raptor in mTORC1 (1,2). These interactions are both necessary and sufficient for mTORC1 activation in response to amino acid signals (1,2).