A gradient of the maternal morphogen dorsal establishes asymmetric patterns of gene expression along the dorsal-ventral axis of early embryos and activates the regulatory genes twist and snail, which are responsible for the differentiation of the ventral mesoderm. Expression of snail is restricted to the presumptive mesoderm, and the sharp lateral limits of this expression help to define the mesoderm-neuroectoderm boundary by repressing the expression of regulatory genes that are responsible for the differentiation of the neuroectoderm. The snail gene encodes a zinc finger protein, and neuroectodermal genes that are normally restricted to ventral-lateral regions of early embryos are expressed throughout ventral regions of snail- mutants. The formation of the sharp snail border involves dosage-sensitive interactions between dorsal and twist, which encode regulatory proteins that are related to the mammalian transcription factors NF-kB and MyoD, respectively.
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