ABCC6 and sickle cell disease: It has also been suggested that oxidative stress is a pathophysiologic factor in PXE because (i) some PXE patients display biochemical signs of oxidative stress [87], (ii) some patients with β-thalassemia or sickle cell anemia – both conditions in which systemic free radical levels are elevated - can display PXE-like manifestations [88–91], and (iii) oxidative stress inhibits expression of ABCC6 gene in human cell lines.