CYP19A1 and breast cancer: However, it is well known that melatonin regulates the synthesis and transformation of androgens and active estrogens in human breast cancer cells and endothelial cells, through the inhibition of aromatase, sulfatase and 17β-HSD1 activity and expression (Cos et al., 2008; Alvarez-García et al., 2013b; Alonso-González et al., 2016).