Fig. 8. Genetic cascades in the anterior PSM regulating somitogenesis. (A) Schematic of the positive (red line) and negative (blue line) regulation surrounding Mesp2. The transcription of Mesp2 is enhanced by both Notch signaling and Tbx6. At the same time, Mesp2 suppresses Notch signaling by activating Lfng and suppressing Dll1 expression. Mesp2 proteins are also rapidly degraded via a proteasome-dependent pathway. We herein propose a new negative regulatory system for Mesp2 via Ripply2. (B) Schematic illustrating how the rostrocaudal polarity is established or disrupted in the anterior PSM of the wild type and Ripply2^-/- mutants. In the anterior PSM, Mesp2 is localized in the rostral compartment of S-1 and suppresses Notch signaling through the suppression of Dll1. By contrast, in the caudal compartment of S0, both Dll1 expression and Notch signaling are retained because of the lack of Mesp2. In the Ripply2^-/- embryo, Mesp2 expression persists for a longer period in both the rostral and caudal compartments, although the suppression on Notch signaling is incomplete. This results in the expansion of the rostral properties within the somites.