ACVRL1 and hereditary hemorrhagic telangiectasia: Two main loci have been identified as the genes responsible for about 85% of the cases in the HHT disease: endoglin (ENG; 9q34) [4] mutated in HHT type 1 (MIM #131195) and the activin receptor-like kinase-1 (ALK1 or ACVRL1; 12q13) [5] affected in HHT type 2 (MIM #601284).