ACVRL1 and hereditary hemorrhagic telangiectasia: Hereditary Hemorrhagic Telangiectasia (HHT), also known as Osler-Weber-Rendu disease, is an autosomal-dominant genetic disorder involving genetic defects in two predominant genes: ENG, codifying for endoglin (HHT-1 type) and ACVRL1/ALK1, codifying for activin receptor-like kinase 1 (HHT-2 type); and, much less frequently, in two other genes, such as MADH4/SMAD4 (JP-HHT) or BMP9/GDF2 (HHT-5) (1).