Unlike polyglutamine repeats, which are more commonly studied, PAEs are typically short (less than 20 Ala residues) and cause developmental defects similar to those seen in Arx, suggesting a shared underlying molecular or genetic mechanism for PAE-related disorders (Albrecht and Mundlos, 2005; Messaed and Rouleau, 2009). Here, ARX is linked to pulmonary artery enlargement.