To examine whether HHcy induces GCH1 S-nitrosylation through iNOS-driven nitrosative stress, we used N-acetyl-cysteine (NAC) to block protein S-nitrosylation in vivo (Online Figs. S1C and S1D), which is a clinical drug used for chronic obstructive lung disease [29], and L-N6-1-Iminoethyl-lysine (L-NIL, Online Fig. S1C) to selectively inhibit iNOS. This evidence concerns the gene GCH1 and chronic obstructive pulmonary disease.