Maternal Wnt/β-catenin signaling is essential for the initiation of dorsal-ventral patterning during vertebrate embryogenesis.Maternal β-catenin accumulates in dorsal marginal nuclei during cleavage stages,but its critical target genes essential for dorsalization are silent until mid-blastula transition (MBT).Here we find that zebrafish netl,a guanine exchange factor (GEF),is specificly expressed in dorsal margin blastomeres after MBT,and acts as a zygotic factor to promote the specification of dorsal cell fates.