The spätzle gene encodes a component of the extracellular signaling pathway establishing the dorsal-ventral pattern of the Drosophila embryo.

spätzle is a maternal effect gene required in the signal transduction pathway that establishes the dorsal-ventral pattern of the Drosophila embryo. spätzle acts immediately upstream of the membrane protein Toll in the genetic pathway, suggesting that spätzle could encode the ventrally localized ligand that activates the receptor activity of Toll. The spätzle gene encodes a novel secreted protein that appears to require activation by a proteolytic processing reaction, which is controlled by the genes that act upstream of spätzle in the genetic pathway. We propose that proteolytic processing of the spätzle protein is confined to the ventral side of the embryo and that the localization of processed spätzle determines where the receptor, Toll, is active.