trn transcripts are detected at all stages of development on northern blots with a peak in 3-12hr embryos. By in situ hybridization, they are first detected at embryonic stage 5 in 8 transverse stripes, the seven most posterior of which correspond to the even pair-rule stripes. At stage 8, four longitudinal trn-expressing stripes appear in the ventrolateral region of the embryo. These coincide with the medial and lateral columns of ac expression. At this time, trn is also expressed in patches of cells on the head that will probably give rise to the brain and anterior sense organs. From stage 8 on, a complex and dynamic pattern of expression is observed involving lateral and longitudinal stripes. From stage 9 on, expression is observed in developing PNS cells including the ventral, lateral, and dorsal sense organs. During stages 15 and 16, trn expression is observed transiently in the stomodeum, proctodeum, and in a short region of the hindgut. In stages 16 and 17 expression is observed in the CNS. During larval and pupal stages, trn expression is associated with the developing CNS and PNS. In third instar larval leg, wing, and eye-antennal discs, regions of trn expression coincide with positions of developing sense organs as well as the inner and outer proliferation centers of the optic lobe. In the eye disc, trn transcripts are found in a repeating pattern at the anterior edge of the morphogenetic furrow and in a subset of developing photoreceptor cells posterior to the furrow.

trn protein is expressed broadly in the morphogenetic furrow where it is restricted to the anterior half, and in subsets of ommatidial cells posterior to the furrow. trn protein is localized mostly to the apical membrane of the photoreceptor cells, but is also visible in the basolateral membrane.