Ligand for the EGF receptor (Gurken). Involved in a number of unrelated developmental choices, for example, dorsal-ventral axis formation, glial migration, sensory organ determination, and muscle development. It is required for photoreceptor determination. Interacts with Star via the lumenal domain. Relocalization to the Golgi apparatus and the plasma membrane is mediated by Star. In the presence of Rhomboid, Spitz is found only in the Golgi, not at the cell surface. Expressed throughout the embryo. Proteolytic processing by Rhomboid occurs in the Golgi. Cleavage takes place within the transmembrane domain close to residue 144 and the active growth factor is released. N-glycosylated and O-glycosylated.