Acts partially redundantly with other irx members in neural patterning. Required for formation of the posterior forebrain, midbrain, hindbrain, and to a lesser extent, spinal cord. Both up-regulates and down-regulates gene expression during neural development. Acts early in neural plate development to induce proneural gene expression and specify a neural precursor state. Also up-regulates repressors that prevent neuronal differentiation. Required during at least two stages of pronephros kidney development; during neurula stages, maintains transcription of key renal genes to define the size and identity of the pronephric anlage, probably in part through regulation of bmp-signaling. Subsequently required for proper formation of the intermediate tubule segment of the pronephros. Primarily expressed in the developing central nervous system (CNS). At gastrula stage, expressed in both the superficial and deep layers of the presumptive neural plate with expression spreading to the prospective hindbrain, spinal cord and midbrain-hindbrain junction as neurulation proceeds. Not expressed in the anterior neural plate and CNS expression in the tadpole excludes the forebrain. Outside of the CNS, expressed around the closing blastopore at early gastrula stages and as gastrulation proceeds, expression switches to the anterior lateral plate mesoderm. In tadpoles, expressed in the ectodermal layer of the branchial arches, and in the otic vesicle. Also expressed in specific and overlapping dynamic patterns with irx1 and irx2 during pronephric kidney development. Renal expression begins before segment-specific terminal differentiation in the pronephric anlage at mid-neurula stage, and is later found in proximal tubule PT3 as well as intermediate tubule segments IT1 and IT2, with expression in the kidney being maintained through to the tadpole stage. By a combination of a neural inducing signal (nog/noggin) and a posteriorizing signal (bFGF). Inhibited in the neural plate by foxd5. The anterior limit of expression at the future border between the prethalamus and thalamus is defined by mutual repression with the anterior repressor fezf2, and also by arx. Induced by retinoic acid (RA) during kidney development. Belongs to the TALE/IRO homeobox family.