The dorsal gene product in Drosophila primarily functions as:
Dorsal gene encodes transcription factor containing Rel homology domain homologous to mammalian NF-kB p50 and p65. Dorsal resides cytoplasmically bound to Cactus but translocates ventrally upon Toll activation, where concentration gradient specifies fates: high activates twist and snail for mesoderm, intermediate specifies neuroectoderm, low permits dorsal ectoderm. Its function encompasses DNA binding, activation and repression, not enzymatic catalysis or scaffolding. Receptor function belongs to Toll. Dorsal represents morphogen whose nuclear concentration interpreted through high and low affinity enhancers generates discrete DV thresholds during patterning.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Dorsal as NF-kB transcription factor - DV morphogen gradient action.