Enhancer trap expression is observed in the developing photoreceptors and cone cells in the third instar eye imaginal disc. It is first expressed behind the morphogenetic furrow and is switched on at around the same time as the first neural anitgens. Thus expression occurs in the cells in the order in which they differentiate, R8, then R2 and R5, R3 and R4, R1 and R6, R7, and finally cone cells. In pupae, expression is observed in cone cells and in primary pigment cells. In adults expression is observed in cells which are probably cone cells as well as in a subset of cells in the optic lobe and brain. In embryos, expression is first observed in the cellular blastoderm in dorsal regions at the anterior and posterior ends and in the dorsal ectoderm anlage, in a pair-rule pattern. At stage 9, expression occurs in two rows of ectodermal cells bordering the mesectoderm. As the mesectoderm invaginates, the rows meet and expression occurs in the entire ventral ectoderm. In germ band extended embryos, additional expression is seen in the head region and in segmentally repeated groups of ectodermal cells. Expression is observed in dorsal and lateral epidermal cells located at the segment boundary and in all ventral epidermal cells until stage 16. As the germ band retracts, expression is seen in the midline glial cells.