To evaluate whether melatonin, indolic hormone produced mainly by the pineal gland, could enhance the pro-apoptotic effect of arsenic trioxide (As2O3) in breast cancer, MCF-7 cells were treated with As2O3-plus- melatonin and then the survival, proliferative rate, caspase-3 activity, and mRNA expression level of anti- apoptosis target genes of NF-κB were investigated. The gene discussed is NFKB1; the disease is breast cancer.