How scaling, the process of expanding proportionally, occurs during development is not well understood. Using the Drosophila melanogaster wing as a model to study scaling quantitatively, Affolter and colleagues examined whether the activity gradient of the morphogen Decapentaplegic (DPP) scales during imaginal disc growth. First, they established an imaging method to reliably quantify the spatial and temporal changes in the activity gradients of DPP — measured by the expression domains of phosphorylated MAD (P-MAD) and Brinker (BRK), two targets downstream of DPP signalling. They found that both P-MAD and BRK expression domains, and therefore DPP activity gradients, scaled with the tissue size during disc growth and that the scaling was transmitted to target genes (dad, sal and omb). Furthermore, scaling of the DPP activity gradient requires pentagone (pent), a gene that is repressed by DPP signalling, as pent mutants have narrower expression domains of P-MAD and BRK.