Author SummaryGrowth control mechanisms ensure that organs attain the correct final size, generally averting tumour growth. This control is often linked to spatially confined domains known as organizers (conserved signalling centres), established along the dorsal-ventral and anterior-posterior axes of the organ by the Notch and Hedgehog pathways, respectively. The organizers emit signals that dictate growth, cell fate specification, and differentiation. However, how the distinct organizing signals received are integrated by cells within a growing organ remains a mystery. By studying how Delta-Notch signalling drives tumorigenesis, we identified the conserved microRNA miR-7 as a co-operative element in tumorigenesis mediated by Delta. We found that the cooperation between the microRNA and Delta-Notch pathway converged on the silencing of two obligatory and functionally redundant Hedgehog receptors, interference hedgehog and brother of ihog. Downregulation of other hedgehog pathway genes via RNA interference or genetic mosaics revealed a tumour suppressor role for Hedgehog signalling in the context of the oncogenic Notch pathway. Given the conservation of miR-7, as well as of the Notch and Hedgehog pathways, the conclusions we have drawn from these studies on Drosophila may be applicable to some human cancers.