The epigenetic hypothesis of PAH consists of changes in DNA methylation levels at the superoxide dismutase 2 (SOD2) and granulysin (GNLY) gene loci; histone H1 levels; abnormal expression levels of histone deacetylases (HDACs) and bromodomain-containing protein 4 (BRD4); and dysregulated microRNA networks (miRNAs) [39,40]. This evidence concerns the gene BRD4 and pulmonary arterial hypertension.