Transcription factor involved in somitogenesis and neurogenesis. Required for the maintenance and differentiation of particular elements of the axial skeleton. May act upstream of PAX9. Plays a role in controlling the development of connections of hypothalamic neurons to pituitary elements, allowing central neurons to reach the peripheral blood circulation and to deliver hormones for control of peripheral functions. Expressed in the paraxial mesoderm, in the developing kidney and central nervous system. In the somite, it is restricted to the caudal half of the newly formed somite and sclerotome. In the central nervous system, it is detected in the developing spinal cord, hindbrain, mesencephalon and telencephalon. Expressed in adult and embryonic magnocellular neurons of the hypothalamo-neurohypophysial system. Mice die perinatally and exhibit severe malformations of the axial skeleton. Pedicles of the neural arches and proximal ribs are not formed. In addition, dorsal root ganglia are disorganized. In the hypothalamo-neurohypophysial system, neurons are viable and able to express neuropeptides; however, the connectivity of magnocellular neurons with posterior pituitary elements is compromised. Marker of antero-posterior subdivisions of the somite. Belongs to the paired homeobox family. Unc-4 subfamily.