18w transcript is expressed in the proximal region of the wing disc, and the anterior compartment in leg discs; expression is absent from the wing pouch.

18w is expressed before and throughout salivary gland invagination during embryonic stages 11 and 12. At stage 13, it is no longer expressed in the salivary glands but is detected in salivary gland ducts.

18w is expressed in follicle cells from oogenesis stage 8 until the end of oogenesis. From stage 8 to 10A, 18w accumulates in the main body follicle cells but not in the terminal cells. Expression is also detected in stalk cells. Later, as the main body follicle cells migrate posteriorly over the oocyte, they continue to express 18w, but expression is not detected in the squamous follicle cells left covering the nurse cells. By stage 10B, expression is limited to a large patch of cells on the dorsal anterior side of the oocyte and to the centripetally migrating cells. During stage 11, the pattern is refined to the centripetally migrating cells and anterior cells along the dorsal midline. Expression remains high in cells secreting the collar and in some of the cells that secrete the dorsal appendages that may be the floor-forming cells of the dorsal appendages.

18w expression is enriched in centripetal cells relative to other cells in the egg chamber.

18w transcripts are detected at all stages of development.

18w is expressed in all developmental stages tested. Expression is first observed at the syncytial blastoderm stage in the presumptive hindgut, head regions, and parasegment 2. Shortly after, stripes of expression are seen first in the even numbered parasegments and then in the odd numbered parasegments. By the extended germ band stage, 18w is expressed in 15 transverse stripes with patches of expression corresponding to the 4 head segments and the hindgut as well. 18w expression in the trunk region was shown to overlap the wg and en stripes and spans the parasegmental boundary. At full germ band extension, the stripes begin to fade and expression is observed in segmentally repeated lateral clusters corresponding to the tracheal pits and the segmental grooves. Expression is accentuated in the cells surrounding the anterior and posterior spiracles. Transient expression in the CNS is also seen. Later, 18w expression is observed in the wing and haltere disc primordia, the dorsal vessel, the antennal-maxillary sense organs, the clypeolabrum, the anterior and posterior spiracles, and in the anal plate.

18w transcripts are expressed in all stages of development tested. They are expressed in a complex pattern during embryogenesis. Expression is first observed just prior to cellular blastoderm formation in three stripes in the anterior portion of the embryo and a broader posterior domain. Subsequently, stripes are seen in the central region of the embryo. At late cellular blastoderm, secondary stripes appear leading to the transient expression of 18 stripes during early gastrulation and 20 stripes at stage 10. 18w expression overlaps the wg and en stripes and spans the parasegmental boundary. At maximum germ band extension, expression is observed along the ventral midline in small clusters of cells posterior to each stripe. Staining is also observed in salivary gland placodes, in tracheal pits, and in five areas in the presumptive head region. During germ band retraction, transcripts are less abundant and more restricted. They are observed in the developing tracheal system, the salivary gland anlagen, the anlagen for the anal plate, the posterior spiracles, the clypeolabrum, and in cells that participate in the formation of the dorsal vessel. In stage 15 and 16, 18w expression is also observed in regions of the head, pharynx, stomach, and hindgut.