To address the topic of local estrogen production, we measured the levels of breast cancer, normal tissue, and plasma estrogens, and correlated hormone levels with the expression of hormone-modulating enzymes, including the different steroid dehydrogenases as well as aromatase and sulphokinase/sulphatase levels (Lønning et al. 2009, Haynes et al. 2010). The gene discussed is CYP19A1; the disease is breast cancer.