EN1 and Parkinson disease: (iv) Mouse models with genetic deficiencies in transcription factor genes, such as the engrailed homeobox 1/2 gene (En1/2) and the paired like homeodomain 3 gene (Pitx3), exhibited features of PD, especially dopaminergic neuronal loss in the substantia nigra, but no definite pathogenic gene mutations were found in PD patients (34).