4-oxo-2-Nonenal-d3 (4-ONE-d3) contains three deuterium atoms at the terminal methyl position. It is intended for use as an internal standard for the quantification of 4-ONE by GC- or LC-mass spectrometry (MS). 4-ONE is a lipid peroxidation product derived from oxidized ω-6 polyunsaturated fatty acids such as arachidonic acid and linoleic acid. It exhibits various biological activities such as cytotoxicity, growth inhibiting activity, genotoxicity, and chemotactic activity and has been widely used as a marker of lipid peroxidation. 4-ONE is a more recently identified product of lipid peroxidation. It actively modifies histidine and lysine residues on proteins and causes protein cross-linking. 4-ONE also modifies 2''-deoxy guanosine, further implicating lipid peroxidation in mutagenesis and carcinogenesis.