Hereditary hemorrhagic telangiectasia (HHT) or Osler-Weber-Rendu syndrome patients are carriers of a heterozygous mutation of the activin receptor-like kinase 1 (ACVRL1), Endoglin (ENG) or Mothers against decapentaplegic homolog 4 (SMAD4) gene. HHT involves the Bone Morphogenetic Protein 9 (BMP9)/Activin receptor-Like Kinase 1 (ALK1)-endoglin signalling pathway. BMP9 is a growth factor that binds to ALK1 receptor and to endoglin its co-receptors and physiologically activates Smad signaling pathway. Endothelial cells in HHT patients display half expression of functional ALK1 receptors or endoglin co-receptors or of the transcription factor SMAD4, which should lead to effects on the functions of these cells. The identification of differences in gene expression between endothelial cells from HHT patients and healthy donors will allow the identification of new functions or new target pathways for therapy. Circulating endothelial cells are rare in the bloodstream in adults, but are present in greater quantities in cord blood.