ACVRL1 and hereditary hemorrhagic telangiectasia: Finally, loci on chromosomes 5 and 7, unlinked to ENG, ACVRL1, or SMAD4 mutations, have been associated with subsets of HHT patients and have been referred to as HHT3 and HHT4, respectively (Cole et al., 2005; Bayrak-Toydemir et al., 2006), while HHT features (HHT5) have also been observed in individuals with heterozygous mutations of growth differentiation factor 2 (GDF2), which encodes BMP9 (Wooderchak-Donahue et al., 2013).