ACVRL1 and juvenile polyposis syndrome: Single mutations are detected in Endoglin (ENG; HHT1) (MIM # 131195) [8], Activin Receptor-Like Kinase 1 (ACVRL1/ALK1; HHT2) (MIM # 601284) [9, 10], or MADH4/SMAD4 (JHPT, a combined syndrome of juvenile polyposis and HHT) [11–13].