Acts as a transcriptional activator (By similarity). In conjunction with NKX2-5, may play a role in both pharyngeal and cardiac embryonic development (PubMed:10733590, PubMed:11390666). Not detected in any neonate or adult tissues. Expressed in developing gut endoderm, cardiac progenitors and heart. Expression restricted to the very narrow development period between stages 8.0 and 11.5 dpc. First detected at 8.0 dpc in the endoderm of the lateral walls and lip of the forming foregut pocket, directly juxtaposed to cardiogenic mesoderm. At 8.5 dpc, expressed in the forming pharynx, predominantly in its lateral aspects where pouches will form. From 8.5 to 11.5 dpc, expressed in the ventrolateral endoderm of all forming pouches, as well as in juxtaposed arch ectoderm and mesenchyme. Expression levels decrease as pouches matured. Also expressed in the ventral hindgut endoderm from 9.5 dpc and in a short segment at the foregut-midgut junction spanning the proximal parts of the common bile and pancreatic ducts. No visible phenotype; possibly due to the redundancy with NKX2-6. Mutant mice are viable and fertile. No obvious abnormalities in the caudal pharyngeal pouch derivatives (thymus, parathyroid glands, and thyroid gland), heart and gut. In NKX2-5 and NKX2-6 double mutants, the pharynx does not form properly and the development of the atrium is less advanced. Belongs to the NK-2 homeobox family.