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Degradation of DNA when gamma-irradiated in aqueous solutions containing cysteine can be efficiently enhanced not only with oxygen, but to the same extent also with Cu2+ ions under hypoxic conditions. The result can be explained by 'self-repair' in this system due to recombination of DNA. with RSS.-R intermediates, and repair inhibition by oxygen or copper involving RSS.-R scavenging. It is emphasized that oxygen enhancement in DNA-thiol systems may occur not only by peroxidation, via defect fixation (DNA-O2.) or thiol activation (RS-O2.), but also by the well-established inactivation of RSS.-R by oxygen. There is evidence also from literature data for a correlation between oxygen enhancement and RSS.-R stability, which varies with thiol concentration, pH and thiol structure.