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. Acts early in neural plate development to induce expression of some but not all proneural genes, and specify a neural precursor state. Also up-regulates repressors that prevent neuronal differentiation. Patterns the neuroectoderm in both the anterior/posterior and dorsal/ventral axes. Probably dispensable for pronephric kidney development. First expressed at gastrula stage 10 in a large region of the dorsolateral ectoderm that includes most of the prospective neural plate. Subsequently expressed in two domains, one within the anterior ectoderm (outside the neural plate) and the other within the neural plate. Within the neural plate, expression extends from the midbrain-hindbrain junction up to, and including, the spinal cord. Expressed in the placodes. Also expressed in specific and overlapping dynamic patterns with irx1 and irx3 during pronephric kidney development. Renal expression begins in late neurula stages in the pronephric anlage, and persists in the pronephros where expression is confined to the intermediate tubule segment IT1. By a combination of a neural inducing signal (nog/noggin) and a posteriorizing signal (retinoic acid). Inhibited in the neural plate by foxd5. Belongs to the TALE/IRO homeobox family.