An important disease mechanism in acquired and idiopathic PAH is downregulation of the enzyme dimethylarginine dimethylaminohydrolase (Ddah, encoded by the genes Ddah1 and Ddah2 in mouse) which eliminates dimethylarginines, including asymmetric dimethylarginine (ADMA), from the circulation (Gorenflo et al. 2001; Arrigoni et al. 2003; Millatt et al. 2003; Kielstein et al. 2005). The gene discussed is DDAH2; the disease is pulmonary arterial hypertension.