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Germ layer formation is a critical step for normal development of vertebrate embryos,which is controlled by coordinated action of multiple signals.It has been well accepted that Nodal signals play an essential role in induction of mesoderm and endoderm.This role is executed by zygotically expressed Nodal proteins.Studies in Xenopus indicate that zygotic expression of Nodal genes is activated by maternally supplied factors,including the T-box transcription factor VegT,the TGF-beta superfamily member Vg1 and the Wnt/beta-catenin pathway components.It remains unknown whether in other vertebrate embryos maternal transcriptional factors are required for induction of zygotic Nodal expression.We found that a maternal transcriptional factor is required for induction of zygotic squint and cyclops,two zebrafish nodal genes,and zygotic mxtx2,a transcriptional factor required for mesendoderm induction.It can directly bind to and activate the transcription activity of these target genes promoters.