The paired-like homeodomain transcription factor-3 (PiTX3) is a member of the homeodomain family of DNA binding proteins known as the RIEG/PITX homeobox gene family. The PiTX (PTX) family of homeobox transcription factors includes PiTX1, PiTX2 and PiTX3. PiTX3 is associated with early ocular development and is later expressed in other organs and limbs. PiTX3 is expressed in the eyes (specifically the developing lens), and has a highly restricted expression pattern in the brain. PiTX3 expression in the midbrain starts just before terminal differentiation of dopaminergic neurons (DA). PiTX3 can act on the promoter of the Tyrosine hydroxylase (TH) gene in a cell-type dependent fashion, suggesting that PiTX3 contributes to the regulation of TH expression in mesDA. A double deletion of the mouse PiTX3 gene causes arrested lens development in the recessive aphakia (ak) mouse mutant, which is characterized by small eyes that lack lenses. Mutations of the PiTX3 gene have been associated with anterior segment dysgenesis and a familial form of congenital cataracts. PiTX3 is required for motor activity and for survival of mesDA. The degeneration of mesDA is the cause of Parkinson’s disease (PD) and results in striatial dopamine deficiency and hypokinetic movement disorder.