NOTCH4 and heritable pulmonary arterial hypertension: HPAH patients with c-terminal (p.H288Y) KLF2 mutation (n = 3) also showed a marked upregulation of Notch4 and ETS-1 in the remodelled pulmonary vasculature, identified by endothelial vWF and a prominent α-SMA staining in the small, normally non-muscularised arterioles (Fig. 7f–i).