PGR and breast cancer: Breast cancer may be defined into three broad clinical subtypes ((hormone receptor (ER/PR)-positive (approx. 70%), HER2-positive (approx. 20%) and triple negative breast cancer (TNBC, approx. 10%)), using laboratory-based immunohistochemistry and/or in situ hybridization assays based on differential expression of oestrogen, progesterone and the HER2 receptor (ER, PR and HER2) (figure 1a).