Nevertheless, comparison of tumour and normal cells highlight oestrogen receptor alpha (ERα) expression in breast cancer, a protein found to be expressed in approximately 20% of all cells in the normal mammary gland (Russo et al, 1999), and 87% of ER-positive cells are luminal epithelial cells or in an intermediate position in the duct wall (Petersen et al, 1987). Here, ESR1 is linked to breast cancer.