Ultrabithorax

thumb|Figure 1 - Mutations in the Ubx gene lead to transformation of appendages in T2 and T3 segments, resulting in a second set of wings observed in the "bithorax" phenotype.|262px Ultrabithorax (Ubx) is a homeobox gene found in insects, and is used in the regulation of patterning in morphogenesis. There are many possible products of this gene, which function as transcription factors. Ubx is used in the specification of serially homologous structures, and is used at many levels of developmental hierarchies. In Drosophila melanogaster it is expressed in the third thoracic (T3) and first abdominal (A1) segments and represses wing formation. The Ubx gene regulates the decisions regarding the number of wings and legs the adult flies will have. The developmental role of the Ubx gene is determined by the splicing of its product, which takes place after translation of the gene. The specific splice factors of a particular cell allow the specific regulation of the developmental fate of that cell, by making different splice variants of transcription factors. In D. melanogaster, at least six different isoforms of Ubx exist.

Mutations of the Ubx gene will lead to transformation of dorsal and ventral appendages of the third thoracic segment (T3), which includes the haltere and third leg, into the counterparts on the second thoracic segment (T2). If Ubx is present in T3, it will prevent the original fate of the T2 segment. Such mutations can produce the second set of wings observed in the bithorax phenotype.