dUTPase is an enzyme found in all organisms that have thymine as a constituent of DNA. Through evolution, humans have two major isoforms of dUTPase: a mitochondrial (mDut) and a nuclear (nDut) isoform. The nuclear isoform of dUTPase is a amino-acids-long protein containing three cysteine residues. nDut's starting methionine is post-translationally cleaved, leaving four unique amino acids. Redox-sensitive cysteine residues are present in the thiolate form under physiological conditions and are therefore prone to redox modifications by H 2 O 2. A wide range of reversible cysteine-based redox modifications are possible, representing a targeted signalling event, which is distinct from irreversible oxidative damage. the catalytically active thiol form of the enzyme (reaction iii, Scheme 2)–22 Importantly, sulfenyl amide formation subse-quently has been observed in other proteins and it has been sug-gested that this posttranslational cysteine modiﬁcation can occur in cells,24 Like other cysteine-based redox switches, the sulfenyl amide.