bbg protein is localized to the apical and lateral sides of gut epithelial cells.

During oogenesis, bbg protein is expressed in the profollicular mesoderm, a layer of follicle cells that separates the germline cyst found at the posterior end of the germarium, from the anterior neighbouring younger cyst; strongly expressed in polar follicle cells; weakly expressed in somatic follicle cells surrounding early egg chambers; in follicle cells surrounding the oocyte; and in the oocyte nucleus. During embryogenesis, bbg protein is predominantly expressed in the developing gut. bbg protein is first detected at embryonic stage 6, in the posterior midgut primordium. bbg protein is expressed in the invaginating hindgut primordium by stage 7, in the anterior midgut primordium at stages 9-10, and in the forgut primordium by stages 11-12. Gut expression is maintained though late embryogenesis, and becomes restricted to the pharynx and proventriculus at late embryonic stages. bbg protein expression in the dorsal ectoderm begins at embryonic stage 6; this expression is restricted to the dorsal edge of the dorsal ectoderm by embryonic stage 9-10. In the wing disc, bbg protein is expressed at the presumptive wing margin, and in a similar pattern in the haltere disc. bbg protein is expressed in a pair of cell clusters near the center of the leg discs; at the medial side of the antennal disc; in a ring-shaped pattern in the larval optic lobe; and at and posterior to the morphogenetic furrow of the eye disc. During pupal eye development, bbg protein is also specifically and dynamically expressed in cone cells, mechanosensory bristle cells, and in secondary and tertiary pigment cells as each cell type becomes specified.