The Drosophila Rel/NF-kB transcription factors ± Dorsal, Dif, and Relish ± control several biological processes, including embryonic pattern formation, muscle development, immunity, and hematopoiesis. Molecular-genetic analysis of 12 mutations that cause embryonic dorsal/ ventral patterning defects has de®ned the steps that control the formation of this axis. Regulated activation of the Toll receptor leads to the establishment of a gradient of nuclear Dorsal protein, which in turn governs the subdivision of the axis and speci®cation of ventral, lateral and dorsal fates. Phenotypic analysis of dorsalventral embryonic mutants and the characterization of the two other¯y Rel proteins, Dif and Relish, have shown that the intracellular portion of the Toll to Cactus pathway also controls the innate immune response in Drosophila. Innate immunity and hematopoiesis are regulated by analogous Rel/NF-kB-family pathways in mammals. The elucidation of the complex regulation and diverse functions of Drosophila Rel proteins underscores the relevance of basic studies in Drosophila.