Updated project metadata. DNMTs catalyze the methylation of cytosine with SAM. DNA methylation affects key cellular processes by regulating gene expression, thus serving a variety of physiological and pathophysiological roles. However, the chemical mechanisms of DNA methylation by which its enzymatic activity is regulated have not been fully elucidated. We found that a critical cysteine residue in DNMT is the target of protein S-nitrosylation, leading to the attenuation of DNMT enzymatic activity and consequent aberrant regulation of gene expression during neoplastic cell proliferation. Our results demonstrate that de novo DNA methylation mediated by DNMT3 is regulated by NO, which is involved in tumor formation.