Plays a role in very early embryogenesis, gastrulation, and the development and subdivision of the diencephalon and the midbrain. Establishes and positions the zona limitans intrathalamica (ZLI) in the diencephalon. May play a role in the organizer function. Probably acts redundantly with otx2 in early brain development. Embryonic expression is highly dynamic. Detected along the marginal zone of symmetric embryos at 5 hours of development and found at the shield, a primary morphological asymmetry at 6 hours of development. With the extension of the body axis, expression extends to neuroectodermal regions fated to become fore- and mid-brain; found in two stripes at the posterior side of the eye rudiments at 12 hours of development, distributed in the diencephalon, midbrain and the epiphysis at 18 hours of development, and found in the diencephalon and the midbrain at 24 hours of development. Belongs to the paired homeobox family. Bicoid subfamily.