Expression of the btl reporter is detected in tracheal branches as well as in a restricted area of adepithelial cells in the third instar wing disc, which subsequently migrate and become the adult tracheal system. These cells do not express htl.

Enhancer trap expression is border cell specific in ovaries. Expression is first detected in stage S9.

Enhancer trap expression is detected at the posterior pole and in two groups of ventral cells at the anterior tip of the embryo just before the onset of gastrulation at stage 5. During gastrulation, expression is detected in the mesectodermal precursor cells as they migrate toward the midline, where they meet to form the midline precursor cells. Expression continues in the midline cells and becomes modulated in a segmental manner. During stage 12, expression is most pronounced in the midline cells in the posterior most half of each segment. This includes the VUM neurons and the posterior pair of midline glial cells.