Following the identification of BMPR2 haploinsufficiency as a molecular mechanism of disease, additional causal variants were described in genes encoding the ALK1 (ACVRL1) and Endoglin (ENG) receptors, which have been observed in association with hereditary haemorrhagic telangiectasia (HHT). This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.