Paired bo, Xenopus/Amphibian, protein Pa, Xenopus/Amphibian,-9 (Pa, Xenopus/Amphibian,-9) is a transcription factor required for normal development of thymus,Porcinearathyroid glands, ultimobranchial bodies, teeth, skeletal elements of skull and laryn, Xenopus/Amphibian, as well as distal limbs. Defects in Pa, Xenopus/Amphibian,-9 are the cause of tooth agenesis selective type 3 (STHAG3), a form of selective tooth agenesis, a common anomaly characterized by the congenital absence of one or more teeth.