Regulates cell determination; development of ommatidia and optic lobe. Is a signaling molecule involved in the process of axon pathfinding in the eye. Part of the Ras pathway regulating programmed cell death in pupal eyes; activated by lozenge (lz). Antagonist for the Egfr receptor (gurken). Inhibits Egfr signaling without interacting directly with the receptor, but instead by sequestering the Egfr-activating ligand spitz (spi). Interacts with spi. During embryogenesis, expression is in a segmental pattern in the ectoderm and in the nervous system. In the eye imaginal disks, expression in photoreceptor cells begins a few rows posterior to the morphogenetic furrow. Also expressed in the wing disk. In the adult, expression is seen in the retina and lamina. The three two-fingered repeats tightly encircle spi, providing the basis for it's sequestration. The first repeat harbors a 120 AA insert specific to drosophilidae. Flies exhibit transformation of the eye mystery cells, which are usually non-neuronal, into extra photoreceptors, and supernumerary cone cells and pigment cells are also recruited. Mutants also exhibit abnormal head involution, a change in a number of sensilla in the antennomaxillary complex and abnormal morphogenesis of the maxillary palp and wings in later stages. Truncated C-terminus.