This region of about 17 cells includes all presumptive mesodermal cells and two rows of ectodermal cells. During germ-band elongation, BIB mRNA is detectable and is maintained in the ectoderm . As prospective neuroblasts delaminate, BIB mRNA is still found as nuclei move basally, but BIB transcript disappears from the neuroblasts after they have completely segregation from the epidermal layer. Toward the end of germ-band extension, mesodermal cells start to express bib .
Big brain is localized along the basolateral cell membranes of neuroectoderm cells.
Concentration of Bib and other proteins may be important for efficient signaling between proneural cells. Malpighian tubules and dorsal vessel Bib protein expression is observed in all of these tissues or their precursors. Bib expression is also observed in the anterior and posterior midgut invaginations , ventral midline cells and tracheal pit cells. While the midgut, ventral midline cells and tracheae are not defective in bib mutant embryos, they are all disrupted by loss of other neurogenic gene function. Conspicuous Bib expression is observed in the adult muscle precursors and in subsets of the larval ventral nerve cord and brain lobes.