Here, upregulation of (P)-CDK2, CDK4, P-CDK6, and Cyclin D1 at the protein level, and an increase in CDK1 and CyclinA2 mRNA levels could be demonstrated, supporting increased activity of the CDK-induced Rb-E2F pathway in HPASMCs from IPAH patients. This evidence concerns the gene CDK4 and idiopathic pulmonary arterial hypertension.