As we previously showed that melatonin inhibits aromatase activity and expression, both in vitro in culture of human breast cancer cells (Cos et al, 2005) and in vivo in rats bearing DMBA-induced mammary tumours (Cos et al, 2006a), we wanted to expand our understanding of the antiaromatase properties of melatonin; therefore, our objective now was to assess which promoters of the aromatase gene are modified by melatonin. Here, CYP19A1 is linked to breast carcinoma.