Heterozygous mutations in endoglin (ENG) and activin receptor-like kinase 1 (ACVRL1; ALK1) genes cause HHT type 1 (HHT1) [4] and HHT type 2 (HHT2) [5], respectively. The gene discussed is ACVRL1; the disease is telangiectasia, hereditary hemorrhagic, type 1.