PECAM1 and pulmonary arterial hypertension: In addition, Hopper et al. have observed that BMPR2 expression reduction in pulmonary artery endothelial cells (using siRNA in vitro and EC-specific Bmpr2 knockout mice in vivo) triggers an increase in High Mobility Group AT-hook 1 (HMGA1) factor and smooth muscle cell protein expressions (α-SMA, SM22α, h1-calponin), and a decrease in CD31 expression, meaning that EndMT is induced by the loss of BMPR2 requiring the HMGA1 factor in PAH [63].