The large family of Hox transcription factors has critical roles in early morphological development and later cell differentiation (McGinnis and Krumlauf 1992;Krumlauf 1994;Lawrence et al. 1996;Magli et al. 1997). Humans have 39 Hox genes arranged in four clusters (HoxAHoxD). The linear arrangement of genes within each cluster facilitates controlled spatial and temporal expression along the anterior-posterior axis of the body (Fig. 1A). Expression of Hox genes in the wrong segment at the wrong time often results in homeotic transformation of that segment (Lamka et al. 1992;Morgan et al. 1992;Duboule 1994a). Hox homeodomains contain identical DNA-base-contacting residues (Fig. 1B) and have very similar DNA sequence specificity, generally preferring a core DNA-binding site containing the sequence 5Ј-TTNAT-3Ј, whose third base depends on the particular Hox protein (Laughon 1991). Cooperative DNA binding with other protein cofactors, such as the PBC family of homeodomain proteins, is thought to enhance the specificity of Hox proteins and thereby allow them to carry out distinct functions (Chan et al. 1994;Chang et al. 1995;Mann and Chan 1996). The PBC family, which includes human Pbx1 and Drosophila Exd, belongs to a class of proteins that contains the TALE (three-aminoacid loop extension)-type homeodomain, so named because of a three-amino-acid insertion between homeodomain residues 23 and 24 in the loop between helices 1 and 2 (Mann 1995;Mann and Chan 1996;Bü rglin 1997). A subset of vertebrate Hox proteins interacts with Pbx1 via a conserved six-amino-acid motif, or hexapeptide, enabling cooperative DNA binding (Chang et al. 1995;Neuteboom et al. 1995;Peltenburg and Murre 1996;Shen et al. 1996Shen et al. , 1997Passner et al. 1999;Piper et al. 1999).Hox proteins required for anterior development are expressed earliest and are restricted to anterior domains. The more posterior proteins are expressed later in development and in more posterior domains (Duboule 1994b;Krumlauf 1994). This character is critical to their in vivo function. In general, when posterior proteins are ex-