The transformation of an unpatterned epithelium into a patterned one is a fundamental issue in morphogenesis. This transformation occurs in a dramatic fashion in the developing eye imaginal disc, the primordium of the Drosophila compound eye. Molecular and developmental analyses reveals that the sine oculis (so) locus encodes a homeodomain-containing protein that is expressed and required in the unpatterned epithelium prior to morphogenesis. In mutants, cells undergo apoptosis. These findings argue that so plays an essential role in controlling the initial events of pattern formation in the eye disc. So is also expressed and required for the development of the rest of the fly visual system, including the optic lobes (i.e., those regions of the brain that process visual information). So is expressed in the optic lobe primordium prior to its invagination from the embryonic ectoderm; in so mutants, the optic lobe primordium fails to invaginate.