To investigate the role of Notch in specifying the dorsal midline structures, the floor plate (FP) and notochord (NC), Andrés Carrasco and colleagues blocked and activated Notch signalling in early frog embryos. Their findings, on p. 2225,show that when Notch signalling is activated, sonic hedgehog(shh) and pintallavis (plvs) expression is stimulated in the FP, whereas that of the NC markers chordin(chd) and brachyury (bra) is repressed. These expression changes are accompanied by FP expansion and a reduction in NC size. From these and other findings, the authors propose that Notch activation favours FP development at the expense of that of the NC in cells in the early organiser that have the potential to develop into either of these structures. This cell-fate switch probably occurs before mid gastrula and requires Presenilin.