PGR and breast cancer: The ErbB-2 proto-oncogene is amplified, overexpressed, and/or hyperactive in 15–25% of patients with breast cancers, such as the ‘HER-2 positive subtype’, which is oestrogen receptor α (ERα) and progesterone receptor (PR) negative, or a subset of the ‘luminal B subtype’, which is ERα/PR positive (Slamon et al, 1989; Menendez et al, 2006).