ACVRL1 and juvenile polyposis syndrome: Endoglin (ENG), activin A receptor type II-like 1 (ACVRL1/ALK1), and SMAD4 mutations cause HHT1 (OMIM 187300), HHT2 (OMIM 600376), and the combined Juvenile Polyposis/HHT (JP/HHT) syndrome (OMIM 175050), respectively [3,4,5].