Han et al. shed new light on the interplay between mechanisms of development and of regeneration (see p. 5123). The genes Msx1 and Msx2 are implicated in early development of the mouse limb, but their roles during limb regeneration have not previously been characterised. Ken Muneoka's lab has shown before that Msx1 is expressed during digit tip regeneration in fetal and neonatal mice, and, by using Msx1 mutants, they now demonstrate that Msx1 is required for this regeneration response. Using a novel in vitro system, they demonstrate that exogenous Bmp4, a protein essential for digit regeneration,rescues the Msx1 mutant phenotype. They also report that endogenous Bmp4 is regulated by the combined activities of Msx1 and Msx2, and that this regulation may be mediated by the transcriptional regulator Runx2.