Evidence to support this hypothesis includes: 1) a millionfold higher STS activity than aromatase activity in liver as well as normal and malignant breast tissues,[1] 2) the origin of oestrone (E1) from oestrone sulfate (E1S) in breast cancer tissue is ∼10-fold greater than that from androstenedione,[2] and 3) STS expression is an important prognostic factor in human breast carcinoma.[3, 4] Most oestrogens that originate from the aromatase pathway are converted into and stored in the body as sulfate conjugates that per se are biologically inactive. This evidence concerns the gene CYP19A1 and breast cancer.