Tumour suppressors and proto-oncogenes play fundamental roles in controlling tissue size, shape and organization. It is generally thought that their deleterious effects on tissue development and homeostasis are associated with defects in cell division, but now (p. 623) Yohanns Bellaïche and colleagues reveal mechanical roles for these genes in Drosophila epithelia. They use time-lapse imaging to follow cell behaviour and dynamics in clones of cells that are mutant for the tumour suppressor Fat (Ft). This analysis reveals that Ft mutant clones round up and reduce their cell-cell contacts with surrounding wild-type tissue in the absence of concomitant cell division and over-proliferation. The authors further show that the loss of Ft activity leads to increased levels of the myosin Dachs within clones and the accumulation of Dachs at clone boundaries. Using laser ablation approaches to probe junctional tension, the authors reveal that this polarized distribution of Dachs at clone boundaries increases junctional tension, whereas Dachs accumulation within the clone body decreases tension; these two activities cooperate to promote clone rounding. These findings, together with the analyses of other proto-oncogenes such as Yorkie, Myc and Ras, point to a novel and key function of tumour suppressors and proto-oncogenes.