Impaired inactivation of RA activities in the nascent somites of Gdf11−/− embryos during E9.5 - E10 stages. RA activities visualized by X-gal staining have no apparent difference between Gdf11+/− (A, A’) and Gdf11−/− (B, B’) embryos at E9.5. RA activities were shown to be shifted to the rostral direction in Gdf11+/− (C, C’) but not in Gdf11−/− embryos (D, D’) at E10.0. Asterisks and arrows indicate the posterior-most X-gal positive somite and MesP2 expression, respectively.