Endothelial cells within chronic pulmonary artery thrombi in CTEPH overexpress transmembrane protein 100 (TMEM100), an activin A receptor-like kinase 1 (ACVRL1 or ALK1) signaling-dependent gene, and TGFβ1 upregulated TMEM100 transcription in healthy lung ECs. Here, TGFB1 is linked to chronic thromboembolic pulmonary hypertension.