Although more than 800 pathogenic variants in more than five genes have been reported to be associated with HHT, variants in endoglin (ENG; located on chromosome 9q34.11) and activin A receptor type II-like 1 (ACVRL1; located on chromosome 12q13.13) genes are present in more than 90% of the patients that undergo molecular testing. This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.