The specificities of Sex combs reduced and Antennapedia are defined by a distinct portion of each protein that includes the homeodomain.

The sequence requirements for distinguishing the functional specificities of two homeodomain proteins, Antennapedia and Sex combs reduced, involved in the specification of segmental identities in Drosophila, have been determined. A series of deletions and hybrid proteins was generated and assayed for their function in vivo after heat shock-induced ectopic expression during development. A distinct portion of each protein, including the residues within and adjacent to both ends of the homeodomain, has been found to almost entirely determine its functional specificity as measured by diagnostic cuticular transformations of embryonic and adult head structures. The remaining sequences contribute to the potency with which the proteins act in different cells and are to a limited extent functionally transferable from one protein to the other.