The neural crest is a remarkable embryonic population of cells found only in vertebrates and has the potential to give rise to many different cell types contributing throughout the body. These derivatives range from the mesenchymal bone and cartilage comprising the facial skeleton, to neuronal derivatives of the peripheral sensory and autonomic nervous systems, to melanocytes throughout the body, and to smooth muscle of the great arteries of the heart. For these cells to correctly progress from an unspecifi ed, nonmigratory population to a wide array of dynamic, differentiated cell types-some...