Previous epidemiological studies have associated circulating sex hormones (oestrogens and androgens) and the growth factor insulin-like growth factor-1 (IGF-1) with an increased risk of breast cancer.5–8 The results for sex hormones have been less conclusive in pre-menopausal women due to fewer data being available in this group, and it is not clear if the associations differ by menopausal status. The gene discussed is IGF1; the disease is breast cancer.