ACVRL1 and juvenile polyposis syndrome: HHT type 1 involves a mutation in the ENG gene coding for endoglin on chromosome 9, HHT type 2 is associated with mutations in the ACVRL1 gene coding for activin receptor-like kinase (ALK) on chromosome 12, and a combined disorder involving juvenile polyposis and HHT is linked to mutations in the MADH4 gene that codes for the transcription factor SMAD4 [7,8].