Endothelial dysfunction appears to be of relevance in TAO,[5–7] and an increase in the level of endothelin 1 (ET-1)—a potent vasoconstrictor—has been observed in patients with clinically active disease and necrotic lesions.[8] Based on these findings, bosentan—a competitive antagonist of ET-1 at the endothelin-A (ET-A) and endothelin-B (ET-B) receptors—is beginning to be used in severe and refractory cases of TAO, and has effectiveness and a good safety profile.[9–15]. This evidence concerns the gene EDN1 and endothelial dysfunction.