Transcriptional regulator which is important for the differentiation and maintenance of meso-diencephalic dopaminergic (mdDA) neurons during development. In addition to its importance during development, it also has roles in the long-term survival and maintenance of the mdDA neurons. Activates NR4A2/NURR1-mediated transcription of genes such as SLC6A3, SLC18A2, TH and DRD2 which are essential for development of mdDA neurons. Acts by decreasing the interaction of NR4A2/NURR1 with the corepressor NCOR2/SMRT which acts through histone deacetylases (HDACs) to keep promoters of NR4A2/NURR1 target genes in a repressed deacetylated state. Essential for the normal lens development and differentiation. Plays a critical role in the maintenance of mitotic activity of lens epithelial cells, fiber cell differentiation and in the control of the temporal and spatial activation of fiber cell-specific crystallins. Positively regulates FOXE3 expression and negatively regulates PROX1 in the anterior lens epithelium, preventing activation of CDKN1B/P27Kip1 and CDKN1C/P57Kip2 and thus maintains lens epithelial cells in cell cycle. Interacts with SFPQ. Highly expressed in developing eye lens. Expression is restricted to the substantia nigra and ventral tegmental area in the midbrain. First visible in 10.5 dpc embryos where expression is confined to the lens vesicles. Between 11.5 dpc and 12.5 dpc, expressed in both the lens epithelium and differentiating primary fiber cells. In the late fetal stage after the lens is formed, primarily found in the lens epithelium and the lens equator region where lens epithelial cells exit from the cell cycle and differentiate into fiber cells (at protein level). First expressed in the eye at 10 dpc embryos. Throughout eye development, expressed in the lens placode and forming lens pit. From 12 dpc, also detected in the midbrain region, tongue, incisor primordia, condensing mesenchyme around the sternum and vertebrae and in the head muscles. Mutations in Pitx3 appear to be the cause of the aphakia (ak) phenotype, a recessive homozygous disease characterized by small eyes and closed eyelids. Mice show loss of nascent substantia nigra dopaminergic neurons at the beginning of their final differentiation and a loss of tyrosine hydroxylase (TH) expression specifically in the substantia nigra neurons. Belongs to the paired homeobox family. Bicoid subfamily.