CDK4 and idiopathic pulmonary arterial hypertension: Although we focused primarily on the differential CDK activity in HPASMCs due to our initial data obtained by peptide-based kinase activity profiling, we could detect expression of CDK2, CDK4 and CDK6 also in other pulmonary vascular cell types (i.e., endothelial and immune cells, adventitial fibroblasts) in the vasculature of both donor and IPAH patients.