This family of proteins are nerve growth factor-like ligands required in the pathway that establishes the dorsal-ventral pattern of the embryo [1]. They form a cystine knot structure [2].

1: The spatzle gene encodes a component of the extracellular signaling pathway establishing the dorsal-ventral pattern of the Drosophila embryo. Morisato D, Anderson KV; Cell. 1994;76:677-688. PMID:8124709

2: Biophysical characterization of refolded Drosophila Spatzle, a cystine knot protein, reveals distinct properties of three isoforms. Hoffmann A, Funkner A, Neumann P, Juhnke S, Walther M, Schierhorn A, Weininger U, Balbach J, Reuter G, Stubbs MT; J Biol Chem. 2008;283:32598-32609. PMID:18790733