Some proteins have very different functions in different subcellular locations. Mitogen-activated protein kinase (MAPK), for example, is usually transported into the nucleus immediately after it is phosphorylated and, once there, it promotes cell proliferation. But, report Marenda et al., in the differentiating vein cells of developing Drosophila wings,phosphorylated MAPK (pMAPK) is held in the cytoplasm, where it controls cell fate (see p. 43). At the same time, note the researchers, pMAPK moves into the nuclei of other wing cells and promotes cell proliferation. Thus, MAPK phosphorylation can signal two different cellular outcomes in developing fly wings - differentiation or proliferation - based on the subcellular localization of pMAPK. Bifurcation of the Ras signalling pathway through this holding of pMAPK in the cytoplasm,suggest the authors, may be generally required for the cell cycle arrest that precedes differentiation.