CYP19A1 and breast carcinoma: 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.