tll and ems are expressed in adjacent regions during the blastoderm stage and during head neurectoderm and brain formation.

At embryonic stage 10, tll expression overlaps with the anterior part of the visual system that will give rise to the anterior lip of the optic lobe. At stage 12, tll is also observed at the posterior lip of the optic lobe but not in Bolwig's organ. In larvae, tll is expressed in the optic lobe of the larva brain. It is also expressed in both the antennal and eye portions of the eye-antennal disc

tll transcripts are expressed in the procephalic neurectoderm from the blastoderm stage into late stages of embryogenesis. tll transcripts are present in all proneural domains of the protocerebrum. The level of tll transcripts in a given domain is highest just before and during the stage in which the neurons from that domain delaminate. From stage 12 on, tll transcripts are present in the optic lobe and are also observed in the late third instar larval optic lobe.

The posterior tll expression domain is severely reduced in csw mutants.

tll transcripts are first observed at nuclear cycle 9 as two dots of staining in the nuclei of the terminal region cells. Staining is strongest in the most terminal nuclei and becomes progressively less intense in subterminal nuclei. During nuclear cycles 10, 11, and 12, staining increases over the nuclei and then in the surrounding cytoplasm until a solid cap of staining is seen at both termini. After the terminal caps form, they resolve into smaller domains. By the beginning of cellularization, the posterior cap has retracted from 20% to 16% egg length. The anterior cap also retracts both from the anterior tip progressing toward the posterior and from the ventral midline progressing laterally. By the end of syncytial blastoderm stage, the anterior domain has become a horseshoe shaped stripe from 75-88% egg length extending about 2/3 of the way towards the ventral midline. Posterior staining is stronger than anterior staining.

tll transcript levels reach a peak at 2-4 hr of embryogenesis. At the syncytial blastoderm stage, tll is expressed in a symmetrical pattern extending from 0-20% egg length and 80-100% egg length. By the cellular blastoderm stage, the tll domain has become smaller. Anteriorly, expression is seen in a stripe from 76-89% egg length in the lateral and dorsal part of the embryo. The posterior band has receded to cover 0-15% egg length. After the cellular blastoderm stage, tll expression is seen mainly in the anterior of the embryo, particularly in the forming brain. From stage 13 on, expression is lost in parts of the brain but persists in cortical regions. By stage 16, expression is mainly seen in the optic lobes. Transient expression occurs in the PNS in stages 12 and 13.

tll-protein expression can be found in neoroepithelial cells in the proximal and upper distal part of the inner proliferation zone in the larval optic anlage. Expression can also be detected in ase-positive ganglion mother cells and transiently in their progeny part of the lobula plate anlage.

tll-protein expression detected immunohistochemically with two different antibodies, one raised against Drosophila Tll and one against human NR2E1.

tll protein is expressed in the oldest of the larval medulla forming neuroblasts.

tll expression is initially found in almost all procephalic neuroblasts, but by embryonic stage 16 it becomes largely restricted to the anterior procephalic neuroblasts. In larvae tll is expressed in the lamina precursor cells, but disappears by the end of the larval stage.

tll protein is observed in syncytial blastoderm embryos as a covering over the anterior and posterior termini of the embryo. As with the RNA, the terminal caps are resolved into smaller domains in the cellular blastoderm embryo. Staining is greater in the posterior cap than in the anterior cap.