ABSTRACT Maternal genes involved in dorsoventral (D/V) patterning of the Drosophila embryo interact to establish a stable nuclear concentration gradient of the Dorsal protein which acts as the morphogen along this axis. This protein belongs to the rel proto-oncogene and NF-KB transcriptional factor family and acts by controlling zygotic gene expression. In the ventral part of the embryo, dorsal specifically activates transcription of the gene twist and ventrally and laterally dorsal represses the expression of zerkn üllt, a gene involved in the formation of dorsal derivatives. The extent of dorsal action is closely related to the affinity and the number of dorsal response elements present in these zygotic gene promoters. twist is one of the first zygotic genes necessary for mesoderm formation. It codes for a ‘b-HLH’ DNA-bind-ing protein which can dimerize and bind to DNA in vitro and to polytene chromosomes in vivo. In addition, in cultured cells twist has been shown to be a transcriptional activator. Thus, the first events of embryonic development along the D/V axis are controlled at the transcriptional level.