optix is a new member of the Six/so gene family from Drosophila that contains both a six domain and a homeodomain. Because of its high amino acid sequence similarity with the mouse Six3 gene, optix is considered to be the orthologous gene from Drosophila rather than sine oculis, as previously believed. optix expression was detected in the eye, wing and haltere imaginal discs. Ectopic expression of optix leads to the formation of ectopic eyes suggesting that optix has important functions in eye development. Although optix and sine oculis belong to the same gene family (Six/so) and share a high degree of amino acid sequence identity, there are a number of factors which suggest that their developmental roles are different: (1) the expression patterns of optix and sine oculis are clearly distinct; (2) sine oculis acts downstream of eyeless, whereas optix is expressed independently of eyeless; (3) sine oculis functions synergistically with eyes absent in eye development whereas optix does not; (4) ectopic expression of optix alone, but not of sine oculis can induce ectopic eyes in the antennal disc. These results suggest that optix is involved in eye morphogenesis by an eyeless-independent mechanism.