We report here the genetical and molecular characterization of a new Drosophila zygotic lethal locus, vrille (vri). vri alleles act not only as dominant maternal enhancers of embryonic dorsoventral patterning defects caused by easter and decapentaplegic (dpp) mutations, but also as dominant zygotic enhancers of dpp alleles for phenotypes in wing. The vri gene encodes a new member of the bZIP family of transcription factors closely related to gene 9 of Xenopus laevis, induced by thyroid hormone during the tadpole tail resorption program, and NF-IL3A, a human T cell transcription factor that transactivates the interleukin-3 promoter. NF-IL3A shares 93% similarity and 60% identity with Vri for a stretch of 68 amino acids that includes the bZIP domain. Although all the alleles tested behave like antimorphs, the dominant enhancement is also seen with a nonsense mutation allele that prevents translation of the bZIP domain. Because of the strong dominant enhancement of dpp phenotypes by vri alleles in both embryo and wing, and also the similarity between the wing vein phenotypes caused by the vri and shortvein dpp alleles, we postulate that vri interacts either directly or indirectly with certain components of the dpp (a TGFβ homologue) signal transduction pathway.