Plays a central role in executing Spemann's organizer phenomenon (the dorsal blastopore lip of the early Xenopus laevis gastrula can organize a complete secondary body axis when transplanted to another embryo). At the start of gastrulation, it is found in a patch of cells encompassing 60 degrees of arc on the dorsal marginal zone. Induced by activin (even in the absence of protein synthesis), lithium chloride, nodal/nr-1, nodal2/nr-2 and derriere. Not affected by basic fibroblast growth factor, and repressed by retinoic acid, and by UV light. Belongs to the paired homeobox family. Bicoid subfamily.