dorsal is one of the maternally active dorsal-ventral polarity genes of Drosophila and is homologous to the vertebrate proto-oncogene c-rel. In wild-type embryos, the dorsal protein is found in the cytoplasm during cleavage. After the nuclei migrate to the periphery of the embryo, a ventral-to-dorsal gradient of nuclear dorsal protein is established. The formation of the nuclear gradient is disrupted in mutant embryos from other maternally active dorsal-ventral polarity genes: in dorsalized embryos only cytoplasmic protein is observed, while in ventralized embryos the nuclear gradient is shifted dorsally. My findings suggest that nuclear localization is critical for dorsal to function as a morphogen and that the distribution of the dorsal protein determines cell fate along the dorsal-ventral axis.