This is because it has also been shown that the luminal A (ERα/PR+, HER-2−) subtype of breast tumours, which represents ∼70% of breast cancers, might, after becoming resistant to antihormonal therapy such as tamoxifen (Benz et al, 1992; Osipo et al, 2003, 2005a) or aromatase inhibitors (Brodie et al, 2009), evolve to overexpress ErbB-2. This evidence concerns the gene PGR and breast cancer.