Planarians (flatworms) can regenerate from irregularly shaped fragments. During regeneration, new tissue forms at the wound and existing tissues are remodelled to make a complete, symmetrical animal. How is this amazing feat achieved? On p. 4043,Alejandro Sánchez Alvarado's group report that BMP signalling regulates several aspects of regeneration in the planarian Schmidtea mediterranea. The researchers use RNAi knockdown to investigate the role of smedolloid-1 (a BMP1/Tolloid-like gene), smedsmad4-1 (a SMAD4-like gene) and smedbmp4-1 (a BMP2/4/DPP-like gene) in regeneration. These experiments show that BMP signalling is involved in the formation and dorsal-ventral (DV) patterning of new tissues at the reset midline of a regenerating fragment. Additional knockdown experiments indicate that BMP signalling also maintains the DV pattern in undamaged adult tissue. These and other results lead the researchers to propose that BMP signalling regulates the dorsal midline of planarians and that the midline has to be reset by BMP activity before injured animals without left-right symmetry can regenerate.