In trials designed to establish the biological activity of fulvestrant, conducted in postmenopausal patients with previously untreated breast cancer, fulvestrant produced significant reductions in cellular levels of the ER, the oestrogen-dependent progesterone receptor (PgR), and the tumour cell proliferation marker Ki67 (DeFriend et al, 1994; Robertson et al, 2001). The gene discussed is PGR; the disease is breast cancer.