In WT mice, downregulation of DDAH1 may be the major cause for elevation of ADMA in hypoxia, while in Ddah1−/− mice, DDAH2 protein is upregulated, limiting further dysregulation of ADMA-related pathways and, by this, pulmonary hypertension and cardiac hypertrophy in hypoxia. Here, DDAH1 is linked to pulmonary hypertension.