Specifically, mutations in ENG (chromosomal locus 9q34.11, encoding the coreceptor endoglin) and ACVRL1 (chromosomal locus 12q13.13, encoding the type I receptor ALK1) are nearly equally responsible for the majority of HHT cases and give rise to the two major forms of the disease, HHT1 (OMIM 187300) and HHT2 (OMIM 600376), respectively (2–4). The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.