However, the 1E1 enzymes, also known as estrogen sulfotransferase (EST1E1) [EC 2.8.2.4] is the one which have the highest affinity for estrogens.16 The estrogen sulfonation is an important feature to protect peripheral tissues from possible excessive estrogenic effect since the addition of the sulfonate group prevents the binding of the estrogen to its receptor.17 Expression of EST1E1 has been found in normal human mammary epithelial cells18,19 and in breast cancer tissue12,20 where it has been associated with an improve prognosis. This evidence concerns the gene SULT1E1 and breast carcinoma.