 Five genetic types of HHT are recognized . Of these , three have been linked to particular genes , while the two remaining have currently only been associated with a particular locus . More than 80 % of all cases of HHT are due to mutations in either ENG or ACVRL1 . A total of over 600 different mutations is known . There is likely to be a predominance of either type in particular populations , but the data are conflicting . MADH4 mutations , which cause colonic polyposis in addition to HHT , comprise about 2 % of disease @-@ causing mutations . Apart from MADH4 , it is not clear whether mutations in ENG and ACVRL1 lead to particular symptoms , although some reports suggest that ENG mutations are more likely to cause lung problems while ACVRL1 mutations may cause more liver problems , and pulmonary hypertension may be a particular problem in people with ACVRL1 mutations . People with exactly the same mutations may have different nature and severity of symptoms , suggesting that additional genes or other risk factors may determine the rate at which lesions develop ; these have not yet been identified . 
