CYP19A1 and breast cancer: Using MCF-7 human breast cancer cells in culture, which express aromatase (Zhou et al, 1993; Sonne-Hansen and Lykkesfeldt, 2005) and the membrane-bound Gi protein-coupled receptor (MT1) melatonin receptor (Ram et al, 1998, 2002), our group has previously shown that melatonin, at physiological concentrations (1 nM), reduces aromatase activity in these cells, both under basal conditions and when aromatase activity is stimulated by cAMP or cortisol (Cos et al, 2005).