The Mix family of homeobox genes--key regulators of mesendoderm formation during vertebrate development.

The Mix/Bix family of paired-like homeobox genes encode evolutionarily conserved, sequence specific, DNA-binding transcription factors that have been implicated in the co-ordination of gene expression, axis formation and cell fate determination during gastrulation in vertebrates. When mutated, these genes give rise to dramatic phenotypes in amphibians, zebrafish and mice, that can be traced back to defects in the formation and specification of mesoderm and endoderm. We review here the biochemical properties of the Mix/Bix proteins and summarise genetic, molecular and embryological studies of Mix/Bix function in mesendoderm development. We emphasise recent data generated using embryonic stem cell differentiation systems that have provided important new insights into Mix/Bix function and the biological roles of these proteins in regulating the earliest phases of vertebrate development.