SMAD9 and pulmonary arterial hypertension: Furthermore, somatic mutations, single nucleotide polymorphisms, microsatellite instability, and aneuploidy have also recently been found to increase predisposition to PAH.3,7,8 For example, two sisters with heritable PAH carry the same missense mutation in Krüppel-like factor 2 (KLF2), a gene known to participate in nitric oxide vascular homeostasis.9 In another PAH patient carrying a BMPRII mutation, Aldred et al.7 identified a deletion in chromosome 13 resulting in the deletion of mothers against decapentaplegic homolog 8 (Smad-8).