DEVELOPMENT Cells initiate and are subject to a great many morphogenetic movements—such as migration, stretching, and invagination—during early embryogenesis. The mechanics at play when cells shuffle around may serve not only to get them to the right place at the right time but also to regulate gene expression. Desprat et al. tested this idea using physical means to mimic deformation forces during early gastrulation in Drosophila . In wild-type embryos, the expression of Twist increases when stomodeal cells are compressed during germ band extension. After experimentally eliminating the natural compressing forces by ablating the most dorsal cells, the authors mechanically perturbed the embryos either by using a needle to create a 20-μm indentation or by using magnetic tweezers to apply a force of 60 nN to a ferrofluid injected just before cellularization (and then captured by the newly formed anterodorsal cells). At the molecular level, reproducing stomodeum compression via these mechanical manipulations resulted in the nuclear localization of Armadillo, which led to elevated Twist expression that in turn was necessary for differentiation of the fly midgut. These results demonstrate the potential that the experimental manipulation of tissue deformation holds for the study of molecular and physiological responses. — BAP Dev. Cell 15 , 470 (2008).