Muscle development of vertebrates has been a paradigm of cell differentiation for many years. Three types of muscle are found in the vertebrate body: skele tal, heart and smooth muscle, and there has been a gradient of concern about these different muscle types in the sequence they are mentioned here. Skeletal muscle has received much attention because it can be induced to differentiate in vitro and because of the clinical relevance of myopathies. The discovery of the muscle-specific members of the bHLH and MADS families of transcription factors must be regarded as a breakthrough not only in...