PGR and breast cancer: Although treatments targeting hormone receptors and human epidermal growth factor receptor 2 (HER2) have improved outcomes in oestrogen receptor (ER), progesterone receptor (PR) or HER2 positive cancers, 15−20% of breast cancers cannot be targeted with such therapies because they lack these receptors and are classed as triple negative (Brouckaert et al., 2012).