CXCL13 and idiopathic pulmonary arterial hypertension: The objectives of the present study were (i) to confirm the findings by Perros et al. [9] in patients with IPAH, (ii) to demonstrate whether increased CXCL13 activity can also be found in lungs from patients with CTEPH and (iii) to investigate whether serum levels of CXCL13 might be used as biomarkers of disease severity in these patient populations.