In PAH cells, up-regulation of HDAC1 [137] along with inactivation of Rb and increased expression of E2F transcription factor 1 (E2F1) target genes (i.e., CDK1, CCNA2...) have been shown to promote cell proliferation [138], reminiscent of their roles during lung development. This evidence concerns the gene E2F1 and pulmonary arterial hypertension.