Involved in the development of the organizer region in the gastrula (Hensen node in chicken). Is first expressed in the Koller's sickle (a crescent shaped thickening located at the edge of the posterior marginal zone), and then it is detected in Hensen node which is considered as the chick organizer. Later expression is seen in the anterior most region of the head process (cells that contribute to the prechordal plate at the midline of the pharyngeal mesendoderm). Belongs to the paired homeobox family. Bicoid subfamily.