The gene coding for the seven less ligand and required for the development of the R7 cell in each ommatidium (one of the single eyes forming the compound eye) in the fly eye. More specifically, activation of a membrane receptor tyrosine kinase (seven less) in the R7 precursor by the ligand bride of sevenless present on the surface of R8 triggers a transduction cascade mediated by ras, thereby establishing the R7 fate of this cell.
See Developmental genetics, Ligands, Ras, R7 and R8 photoreceptors, Sevenless, Tyrosine kinase receptor/pathway