Breast cancer displays molecular and histological heterogeneity and can be classified into three main subtypes: tumours expressing hormone receptors (hormone receptor‐positive [HR+]: oestrogen receptor (ER+) or progesterone receptor [PR+]), tumours expressing human epidermal growth factor receptor 2 (HER2+) and triple‐negative breast cancer (TNBC), which lacks ER, PR and HER2 expression (Lehmann et al., 2011; Liedtke et al., 2008). The gene discussed is PGR; the disease is breast cancer.