Enhancer trap expression is first observed during gastrulation at which point the staining is uniform throughout the embryo. In stage 11, stronger localized expression is seen in a few cells in the gnathal segments. In the thoracic and abdominal segments, stronger localized expression is observed in stages 12 and 13. Expression also occurs in a ring of cells bordering the amnioserosa. In stage 15 and later, the staining is again uniform. In third instar larvae, enhancer trap expression is observed in the brain, ventral nerve cord, and male and female gonads. Some staining is also observed in imaginal discs. This is in contrast to the lack of oaf transcripts in these tissues. In adult male testes, expression is observed in the primary and secondary spermatocytes, spermatids and spermatogonia. In ovarioles, staining occurs within the germarium and in nurse cells at all stages. A low level is seen in the maturing oocyte.