In the vast majority of HHT cases, one of two genes, ENG (HHT1) or ACVRL1 (HHT2), is mutated in a single allele, resulting in the haploinsufficiency of the respective heterozygous gene [4,5], leading to a disturbance of normal angiogenesis [6]. The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.