THE pattern of skeletal structures and muscles in the branchial region of the head is profoundly influenced by the neural crest1, whose cells arise at discrete segmental levels of the chick hindbrain: specifically, rhombomeres (r)l + 2, r4 and r6, whereas r3 and r5 are crest-depleted2. We have demonstrated that an interaction between even-numbered rhombomeres and r3/r5 effects this depletion of neural crest, resulting in the sculpting of discrete migratory streams of neural crest3. This mechanism acts through increased expression of msx2 and the induction of apoptosis in dorsal cells of r3 and r5 (ref. 3) (Fig. 1A). Here we demonstrate that the signalling molecule Bmp4 is expressed in r3 and r5 and is dependent on the neighbouring rhombomeres. Addition of recombinant BMP4 protein to explant cultures of r3 or r5, which produce neural crest when isolated from their neighbouring rhombomeres, upregulates msx2 and reinstates apoptosis in the neural crest population.