The developing salivary glands in Df(2R)rib-ex12 mutant animals fail to turn and migrate properly. In addition, the dorsal trunks of the trachea exhibit only partial extension at stage 14.

Stage 13 Df(2R)rib-ex12 embryos show little tracheal branch budding or outgrowth. By stage 14, 99% of tracheal branches remain stalled, although 1% have formed and grown out excessively and in abnormal directions. Other defects, such as failure of dorsal closure and midgut constriction formation are seen by stage 14. Pseudopodia extend from the basal surface of the tracheal epithelium in stage 12 Df(2R)rib-ex12 embryos as occurs in wild type, but they are more numerous and more pronounced than in wild type, occasionally forming extremely long processes that appear to reach their normal targets. This dissociation of the migration of the apical and basal tracheal surfaces is evident at stage 12 and continues for several hours into stage 14.

Based on the extent of salivary gland development, the following rib alleles can be ranked from strongest to weakest as follows: Df(2R)rib-ex12 > rib^P7 > rib¹/rib^P7 > rib¹.

Based on the extent of tracheal development, the following rib alleles can be ranked from strongest to weakest as follows: Df(2R)rib-ex12 > rib¹/rib^P7 > rib¹ > rib^P7 .

The centromere-proximal breakpoint lies between the site of insertion of P{lacZ}Terminal-3 and prod, within 40 nucleotides 5' to genomic sequence 5'-TGGCCAACTCTTCCTC.