Somites, the most obviously segmented structures in vertebrate embryos, are subdivided into anterior (rostral) and posterior (caudal) compartments. Repression and activation of Notch signalling are essential for the establishment of the rostral and caudal compartments of the somite, respectively. The mechanism by which Notch is repressed has remained elusive but, on p. 55, Yumiko Saga and colleagues identify the bHLH transcription factor Mesp2 as a novel negative regulator of Notch signalling in mouse somites. In the absence of Mesp2, somites are completely caudalised but, intriguingly, the researchers now show that the introduction of a dominant-negative form of Rbpj (a downstream effector of Notch signalling) into the Mesp2 locus largely rescues the segmental defects of Mesp2-null mice. They also report that Mesp2 represses Notch signalling independently of its function as a transcription factor by inducing the destabilisation of mastermind-like 1, a core regulator of the Notch signalling pathway. These new findings shed light on the molecular mechanisms that control the rostrocaudal patterning of somites.