The mechanistic target of rapamycin complex I (mTORC1) is a central regulator of 
cellular and organismal growth, and hyperactivation of this pathway is 
implicated in the pathogenesis of many human diseases including cancer and 
diabetes. mTORC1 promotes growth in response to the availability of nutrients, 
such as amino acids, which drive mTORC1 to the lysosomal surface, its site of 
activation. How amino acid levels are communicated to mTORC1 is only recently 
coming to light by the discovery of a lysosome-based signaling system composed 
of Rags (Ras-related GTPases) and Ragulator v-ATPase, GATOR (GAP activity 
towards Rags), and folliculin (FLCN) complexes. Increased understanding of this 
pathway will not only provide insight into growth control but also into the 
human pathologies triggered by its deregulation.