PDGFRB and idiopathic pulmonary arterial hypertension: Plexiform lesions, observed in eight out of nine IPAH cases, showed mild PDGFR-β positivity: in some cases there was only immunoreactivity of endothelium while in other lesions there was immunoreactivity of endothelial and subendothelial stromal cells, with thin lines of positive immunoreactivity demarcating the basal side of endothelial cells (Figure 1H).