HHT is a rare autosomal dominant genetic disorder characterised by arteriovenous malformations and multiple telangiectasias [184]; it is frequently linked to defects in ACVRL1 and ENG and as HHT and PAH may co-present in families, suggests a common molecular aetiology [103,104,105]. Here, ACVRL1 is linked to pulmonary arterial hypertension.