Somite boundaries formed during embryogenesis, at the interface between Notch1-activated and -repressed domains. Genetic and biochemical studies indicate that this interface is generated by suppression of Notch activity by mesoderm posterior 2 (Mesp2) through induction of the lunatic fringe gene (Lfng). It is proposed that the oscillation of Notch activity is arrested and translated in the wavefront by Mesp2.