Defines dorsal-ventral domains in developing brain. May play a role in defining positional information along the anterior-posterior (a/p) axis and the dorsal-ventral (d/v) axis of the developing nervous system. May be involved in determining positional or boundary information rather than determining a given cell type. Forebrain and midbrain. Is first detectable at the neural plate stage (stage 14). Levels gradually increase during later neurula stages, and becomes fairly constant throughout tailbud and hatching stages before declining at late swimming tadpole stages. Belongs to the NK-2 homeobox family.