Required for visual system development. May transcriptionally regulate genes necessary for optic lobe invagination and Bolwig's nerve formation. In developing embryos, expressed in the eye disk epithelium, bolwig's organ and the optic lobe primordium at areas of invagination. In adults, present in photoreceptor cells in the apical regions of the retina, and in optic lobes. In the eye imaginal disk, first expressed at the onset of the third instar and continues throughout this stage. Expression in the optic lobe primordium begins at stage 5 and disappears when invagination is completed (stage 12). Further expression is noted in optic lobe ganglia in late third instar. Flies specifically display visual defects. These range from reduced ocelli and ommatidia number in weak loss-of-function phenotypes to complete absence of compound eyes and bolwig's organ in more severe lethal phenotypes. Belongs to the SIX/Sine oculis homeobox family.