Moreover, melatonin and its metabolites were also documented to exhibit free radical avoidance properties, by downregulating pro-oxidative enzymes like inducible nitric oxide synthase (iNOS) both in vitro (dose-dependent) and in vivo as well as inhibiting the mRNA expression of cyclo-oxygenase 1 (COX-1) and COX-2 in human breast cancer cells (MCF-7) [142,143]. This evidence concerns the gene PTGS1 and breast cancer.