Involved in EGF receptor signaling. Has an early role in photoreceptor development. Interacts with the receptor torpedo in the eye. Interacts with Spitz via the lumenal domain and mediates its transport to the Golgi. In the larval eye disk, expression is first seen at the morphogenetic furrow, then in the developing R2, R5, and R8 cells as well as in the posterior clusters of the disk in additional R cells. Expressed in embryos and larvae.