The screw gene encodes a ubiquitously expressed member of the TGF-beta family required for specification of dorsal cell fates in the Drosophila embryo.

The decapentaplegic (dpp) gene product, a TGF-beta related ligand, acts as an extracellular morphogen to establish at least two cellular response thresholds within the dorsal half of the Drosophila embryo. Null mutations in the screw (scw) gene are phenotypically similar to moderate dpp mutants and cause dorsal cells to adopt ventral fates. We show that scw encodes a novel TGF-beta protein and is an integral part of the signal that specifies dorsal pattern. Although scw is expressed uniformly during blastoderm stages, its effect on development appears graded and is restricted to the dorsal side of the embryo. Our results indicate that DPP activity alone is insufficient to specify different dorsal cell fates. We propose that SCW and DPP act together to establish distinct response boundaries within the dorsal half of the embryo, perhaps by forming heterodimers that have higher activity than homodimers of either molecule alone.