Mammalian EST sulfurylates the hydroxyl group of estrogenic steroids by transferring the sulfate from a cosubstrate adenosine 3 prime-phosphate-5 prime-phosphosulfate. Sulfurylated steroids do not bind to the oestrogen receptor with high affinity and, therefore, are hormonally inactive. EST plays an important role in controlling the intracellular level of the receptor-active estrogens.