Different animal species have characteristic sizes. A mouse does not grow to the size of a horse, for example, because during development, environmental cues (particularly nutrient availability) act together with genetic cues to regulate cell growth and proliferation. The insulin receptor (InR) signalling pathway cell autonomously controls cellular responses to nutrient availability. Now, on p. 2617, Milán and colleagues report that calderón, which encodes a new organic cation transporter of the major facilitator superfamily, is a downstream effector of InR in developing Drosophila tissues. The researchers show that calderón mutant flies are smaller than wild-type flies and developmentally delayed, a phenotype that resembles that caused by mutations in the InR pathway. Genetic experiments indicate that the expression of calderón is positively regulated by InR downstream effectors,including TOR (target of rapamycin), and that calderón is required for TOR-mediated growth induction. Thus, the authors conclude, calderón is required for the cell-autonomous, insulin-mediated control of cell growth and proliferation during Drosophiladevelopment.