The nitrated derivative of cGMP, 8-NO2-cGMP (8-Nitro-cGMP), is considered to play an important role in NO-mediated signal transduction. Formation of protein Cys-cGMP adducts by 8-NO2-cGMP was identified as a new post-translational modification of proteins, which is called S-guanylation. Reference: Sawa et al., Nat. Chem. Biol., 3, 727 - 735 (2007).