Extra specificity from extradenticle: the partnership between HOX and PBX/EXD homeodomain proteins

For many DNA-binding transcription factors it is often difficult to reconcile their highly specific in vivo functions with their less specific in vitro DNA-binding properties. Cooperative DNA binding with cofactors often provides part of the answer to this paradox and recent studies have demonstrated this to be the case for the homeotic complex (HOX) family of transcription factors. However, the unique problem posed by these highly related and developmentally important transcription factors requires additional twists to the standard solution, which are beginning to become apparent from the characterization of the HOX cofactors encoded by the extradenticle and PBX genes.