Ultrabithorax or Ubx is a member of the homeobox gene family. In Drosophila melanogaster it is expresse in the T3 and A1 segments and represses wing formation. The Ubx gene regulates the decisions regarding the number of wings and legs the adult Drosophila melanogaster will have. Ubx is activated when there is a certain amount of Hunchback protein, which is a product of a gap gene. The concentrations of Hunchback only exist in the middle region of the embryo, therefore Ubx is expressed only in these segments as well.