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19 p.-6 fig. Plumitallo. Sara et al., Hereditary Hemorrhagic Telangiectasia (HHT) is a rare disease, with an autosomal dominant inheritance and a worldwide incidence of about 1: 5000 individuals. In >80% of patients, HHT is caused by mutations in either ENG or ACVRL1, which code for ENDOGLIN and Activin A Receptor Type II-Like Kinase 1 (ALK1), belonging to the TGF-β/BMP signalling pathway. Typical HHT clinical features are mucocutaneous telangiectases, arteriovenous malformations, spontaneous and recurrent epistaxis, as well as gastrointestinal bleedings. An additional, but less frequent, clinical manifestation in some HHT patients is the presence of Pulmonary Arterial Hypertension (PAH). The aim of this work is to describe the functional role of a novel ENG intronic variant found in a patient affected by both HHT and PAH, in order to assess whether it has a pathogenic role. We proved that the variant lies in a novel binding-site for the transcription factor Sp1, known to be involved in the regulation of ENG and ACVRL1 transcription. We confirmed a pathogenic role for this intronic variant, as it significantly reduces ENG transcription by affecting this novel Sp1 binding-site., This work was supported by grants from the Ministerio de Industria, Economia y Competitividad (SAF2013-43421-R to CB) and the Centro de Investigacion Biomedica en Red de Enfermedades Raras (CIBERER; ISCIII-CB06/07/0038 and ER16PIAC707 to CB) in Spain, both supported by European Regional Development Funds (FEDER). CIBERER is an initiative of the Instituto de Salud Carlos III (ISCIII) in Spain. This work is part of the results of research activity by SP, who holds a research fellowship (2/R.C./2016, n.20160028774) from the IRCCS Fondazione Policlinico San Matteo, Pavia, Italy.