Understanding the mechanisms through which multicellular organisms regulate 
cell, organ and body growth is of relevance to developmental biology and to 
research on growth-related diseases such as cancer. Here we describe a new 
effector in growth control, the small GTPase Rheb (Ras homologue enriched in 
brain). Mutations in the Drosophila melanogaster Rheb gene were isolated as 
growth-inhibitors, whereas overexpression of Rheb promoted cell growth. Our 
genetic and biochemical analyses suggest that Rheb functions downstream of the 
tumour suppressors Tsc1 (tuberous sclerosis 1)-Tsc2 in the TOR (target of 
rapamycin) signalling pathway to control growth, and that a major effector of 
Rheb function is ribosomal S6 kinase (S6K).