GTPCH1 inhibition leads to a rapid decrease in BH4 level and consequent eNOS uncoupling.6 GTPCH1 deficiency is an important mechanism of endothelial dysfunction in cardiovascular diseases and diabetes.7, 8 Considering the vital role of GTPCH1, investigation is needed of the mechanism involved in regulating GTPCH1 gene expression. The gene discussed is GCH1; the disease is cardiovascular disorder.