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7 p.-4 fig., Hereditary hemorrhagic telangiectasia (HHT) is a vascular rare disease characterized by nose and gastrointestinal bleeding, skin and mucosa telangiectasias, and arteriovenous malformations in internal organs. HHT shows an autosomal dominant inheritance and a worldwide prevalence of approximately 1:5000 individuals. In >80% of patients, HHT is caused by mutations in either ENG (HHT1) or ACVRL1 (HHT2) genes, which code for the membrane proteins Endoglin and Activin A Receptor Type II-Like Kinase 1 (ALK1), respectively, both belonging to the TGF-β/BMP signaling pathway. In this work, we describe a novel mutation in exon 9 of ENG (c.1145 G > A) found in five affected members of a family, all of them with characteristic symptoms of HHT. This mutation involves Cys382 residue of the Endoglin protein (p.Cys382 > Tyr) in the zona pellucida (ZP) module of its extracellular region. This is a critical residue involved in a conserved intrachain disulphide bond and in the correct folding of the protein. In fact, transfection studies in human cells using Endoglin expression vectors demonstrated that the p.Cys382 > Tyr mutation results in a marked reduction in the levels of the Endoglin protein. These results demonstrate the pathogenic role for this variant in HHT1 and confirm the key function of Cys382 in Endoglin expression., This work was supported by the Ministerio de Economía, Industria y Competitividad (Grant SAF2013-43421-R to CB); the Consejo Superior de Investigaciones Científicas (Grant 201420E039 and 201920E022 to CB); and the Centro de Investigacion Biomedica en Red de Enfermedades Raras (CIBERER; Grant ISCIII-CB06/07/0038 to CB and contract to LR-L) of Spain. CIBERER is an initiative of the Instituto de Salud Carlos III (ISCIII) of Spain supported by FEDER funds.