Because DNMT1 is the key maintenance methyltransferase, and DNMT3a and DNMT3b enzymes methylate unmethylated and hemimethylated CpG islands, we suggest increased DNMT3a and DNMT3b expression and activity potentially leads to de novo hypermethylation of genes in PAH patients. This evidence concerns the gene DNMT3B and pulmonary arterial hypertension.