The increase in oxidants, the preventive effect on cytotoxicity of antioxidants, the known role of free radicals in activating NF-κB (Schoonbroodt and Piette, 2000) and the prevention of the increase in cell death and Fas/Fas L expression by the prevention of NF-κB activation are in support of the implication of the rise in free radicals in the cytotoxic effect of melatonin in some particular types of tumours. This evidence concerns the gene FAS and neoplasm.