PCNA and idiopathic pulmonary arterial hypertension: Consistent with the data from murine models, we observed increased expression of the αSMA cell type marker and proliferation marker cyclin D1 in isolated human PAs from IPAH patients, corroborating previous observations of increased percentages of SMCs positive for PCNA or Ki‐67 proliferation marker in PAs from IPAH patients 30, 31.