Each of the 17 known homeo box genes shows a unique pattern of expression during early embryonic development. Thus, virtually every embryonic cell contains a unique combination of active and inactive homeo box genes. 2. These different permutations of homeo box gene expression are thought to specify unique positional values, and appear to play key roles in selecting diverse pathways of morphogenesis. 3. It has been proposed that selective patterns of homeo box gene expression involve cross-regulatory interactions. For example, it appears that homeotic genes expressed in posterior regions of the embryo (such as abd-A and Abd-B) repress the expression of those homeotic genes expressed in more anterior regions (such as Antp and Ubx). Such interactions might involve sequence-specific DNA binding and occur at the level of transcription. 4. The establishment of selective patterns of pair-rule gene expression along the anterior-posterior embryonic body axis might involve the differential regulation of these genes by a common set of transcription factors. In particular, it appears that eve and ftz show opposite responses to regulatory products that are either directly or indirectly specified by several gap genes.
