MIXER, A HOMEOBOX GENE REQUIRED FOR ENDODERM DEVELOPMENT

An expression cloning strategy in Xenopus laevis was used to isolate a homeobox-containing gene, Mixer, that can cause embryonic cells to fo rm endoderm. Mixer transcripts are found specifically in the prospecti ve endoderm of gastrula, which coincides with the time and place that endodermal cells become histologically distinct and irreversibly deter mined. Loss-of-function studies with a dominant inhibitory mutant demo nstrate that Mixer activity is required for endoderm development. In p articular, the expression of Sox17 alpha and Sox17 beta, two previousl y identified endodermal determinants, require Mixer function. Together , these data suggest that Mixer is an embryonic transcription factor i nvolved in specifying the endodermal germ layer.