Fs is uniformly distributed in syncytial blastoderm embryos. By the cellular blastoderm stage, it is limited to a dorsal stripe with stronger expression in the anterior and posterior regions. This continues until the onset of germband retraction. At stage 15, expression is detected in the male gonads. In larvae, Fs is prominently expressed in four distinct regions of the brain, in wing and leg discs, and in the eye discs in a stripe of cells anterior to the morphogenetic furrow and in differentiating cells behind the furrow. It is also detected in the developing testis and in nurse cells in the ovary.

Fs transcript has a distribution in early embryos consistant with maternal deposition. At the beginning of germ band extenstion, Fs transcript accumulates in a stripe along the dorsal surface of the embryo, particularly robustly along the path of germ cell migration. In late embryos, Fs transcript is expressed in the somatic cells of male, but not female, gonads. During the third larval instar, Fs transcript is widely distributed in the lobes of the larval brain, but not in the rest of the central nervous system. Transcript is also robustly expressed in all imaginal discs. In the male germ line, Fs transcript is expressed in primary spermatocytes in both larval and adult testes. Fs is ubiquitously expressed at a low level in somatic and germ cells at all stages of oogenesis.

Eye-enriched transcripts determined by ratio of expression level in wild-type heads. versus expression level in so heads.