It is now clear that there are two types of drug resistance to tamoxifen: intrinsic resistance, where the tumour is either ER negative or ER positive with pre-existing enhanced survival pathways (HER2/neu plus a coactivator molecule AIB1) (Osborne et al, 2003), and acquired resistance where an ER-positive tumour that initially responds to treatment now becomes tamoxifen-stimulated and grows in response to either tamoxifen or oestrogen. This evidence concerns the gene ERBB2 and neoplasm.