CYP19A1 and breast cancer: The relationships of testosterone (and other androgens) with the development of breast cancer are not well understood; testosterone can be converted to oestradiol by aromatase in adipose tissue and other peripheral tissues including breast tumour cells, and might also have direct effects through the androgen receptor which is commonly present in breast cancer cells.14 We found a moderately large positive association between total and calculated free testosterone and breast cancer risk in post-menopausal women but not in pre-menopausal women.