One of the major endogenus cGMP derivatives, a class of second messengers in the nitric oxide signalling pathway. It modifies the target protein's cysteine residues via a stable sulfide bond through the NO group of the guanine base, in a post translational modification also known as S-guanylation, a process involved in the regulation of the physiological roles of several important proteins. The S-guanylation of tau was shown to block the aggregation process and the subsequent formation of tau oligomers.