Professor Simpson is recognised as the world leader in the field of oestrogen biosynthesis. His group were the first to clone the gene encoding aromatase, the enzyme responsible for oestrogen biosynthesis, and show that tissue-specific regulation was under the control of tissue-specific promoters. This led to the concept that in post-menopausal women oestrogen action in breast, brain and bone is due to local production in these respective sites. This new concept implies that unique modulation of synthesis in a tissue-specific fashion is possible, leading to the development of breast-specific inhibitors of aromatase expression, and thereby preventing the development of breast tumours.