Ligand for the EGF receptor. Seems to play a role in the global proliferation of wing disc cells and the larval patterning. Shows a strong synergistic genetic interaction with spi, suggesting a molecular interdependence. Required for the development of interveins cells. Expressed in blastoderm embryos in two ventrolateral stripes that are brought to the midline as gastrulation proceeds. In the germ-band retraction stage, expression is seen in the CNS and epidermis. At late blastoderm, expression is localized in the anlagen of the amnioserosa. Expression in the head, cypeolabrum, maxillary and labial lobes, and around the stomodeum throughout embryo development. In late embryos, expression decays in all ectodermal cells and appears in the segmental muscles and the gut wall. In the larva, expression occurs in the dorsal metathoracic disc, the eye-antennal disc and the ventral thoracic disc. Found in the intervein in the pupa.