Expressed in specific and overlapping patterns with Irx1 and Irx2 in the developing and adult metanephric kidney. In the adult metanephros, renal expression is found in the loop of Henle in the S3 proximal tubule segment and in the thick ascending limb (TAL) of the distal tubule. First detected in the mesoderm at stage 7.5 dpc. During neurogenesis (9.5 dpc to 10.5 dpc), predominantly expressed along the anteroposterior axis of the CNS in the mesencephalon, metencephalon, rhombencephalon and spinal cord, with expression excluded from the midbrain-hindbrain junction. Beginning at 9.5 dpc, expression is found in the epithelial component of the branchial arches and foregut. At later stages, expressed in somites. At 10.5 dpc, expressed in the cephalic mesenchyme surrounding the optic vesicle. By 12.5 dpc, expression remains in the mesenchyme and also begins in the neuroretina, and at 16.5 dpc expression is exclusively located in the inner neuroblast layers of the neuroretina. Starting at the otic vesicle stage, shows regionalized expression in the developing inner ear with expression in the entire otic vesicle from 10.5 dpc onwards. Expressed in distinct patterns in the developing limb buds from 10.5 dpc onwards. Expressed in the developing heart in the ventricular septum from the onset of its formation (10.5 dpc) onward. In fetal stages, expression becomes confined to the myocardium of the atrioventricular bundle and bundle branches of the forming ventricular conduction system. Belongs to the TALE/IRO homeobox family.