4-Oxo-2-nonenal (4-ONE) is a reactive aldehyde formed during lipid peroxidation, primarily used as a biomarker for oxidative stress in biological systems. It plays a significant role in studying cellular damage associated with aging, neurodegenerative diseases such as Alzheimer's and Parkinson's, and cardiovascular disorders. Due to its high reactivity with proteins, DNA, and phospholipids, 4-ONE is employed in research to investigate protein modification mechanisms, particularly the formation of covalent adducts that alter protein function and contribute to disease pathology. It is also utilized in experimental models to induce oxidative stress and evaluate the protective effects of antioxidants and therapeutic agents. Its presence in tissues and fluids is measured to assess the extent of lipid peroxidation and evaluate the efficacy of interventions targeting oxidative damage.