ACVRL1 and juvenile polyposis syndrome: There are three types of HHT, characterized by mutations in members of the TGFβ/BMP signaling pathway: (1) type 1 results from loss-of-function (LoF) mutations in one copy of endoglin (ENG) [26,27]; (2) type 2 results from LoF mutations in activin A receptor like type 1 (ACVRL1 or ALK1+/−) [28]; and (3) mutations in SMAD4 cause a combined syndrome of juvenile polyposis and HHT (JP-HHT), accounting for 2% of HTT cases [29] (Figure 1).