PGR and breast cancer: Breast cancer is treated based on the receptor status of the tumour, specifically oestrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor-2 (HER2), and the main molecular subtypes are termed Luminal A (ER/PR-positive); Luminal B (ER/PR-positive, higher histological grade than Luminal A); HER2-positive; and triple-negative (ER/PR/HER2-negative) (Cardiff and Kenney, 2011).