Figure 1.

The basic enzymatic mechanisms involved in the transformation of steroids in the normal mammary gland and in breast cancer tissue. Melatonin acts as a selective estrogen enzyme modulators (SEEM) by inhibiting the expression and activity of the enzymes (labeled in red) responsible for the formation of biologically active estrogens from steroids with low biological activity, whereas it increases the expression and activity of the enzymes (labeled in green) involved in the transformation of estrogens into their inactive sulfoconjugates. In breast cancer tissue, the local production of steroids is biased towards production of the more active forms (wide arrows).